 <sect1 id="functions-matching">
  <title>Pattern Matching</title>

  <indexterm zone="functions-matching">
   <primary>pattern matching</primary>
  </indexterm>

   <para>
    There are three separate approaches to pattern matching provided
    by <productname>PostgreSQL</productname>: the traditional
    <acronym>SQL</acronym> <function>LIKE</function> operator, the
    more recent <function>SIMILAR TO</function> operator (added in
    SQL:1999), and <acronym>POSIX</acronym>-style regular
    expressions.  Aside from the basic <quote>does this string match
    this pattern?</quote> operators, functions are available to extract
    or replace matching substrings and to split a string at matching
    locations.
   </para>

   <tip>
    <para>
     If you have pattern matching needs that go beyond this,
     consider writing a user-defined function in Perl or Tcl.
    </para>
   </tip>

   <caution>
    <para>
     While most regular-expression searches can be executed very quickly,
     regular expressions can be contrived that take arbitrary amounts of
     time and memory to process.  Be wary of accepting regular-expression
     search patterns from hostile sources.  If you must do so, it is
     advisable to impose a statement timeout.
    </para>

    <para>
     Searches using <function>SIMILAR TO</function> patterns have the same
     security hazards, since <function>SIMILAR TO</function> provides many
     of the same capabilities as <acronym>POSIX</acronym>-style regular
     expressions.
    </para>

    <para>
     <function>LIKE</function> searches, being much simpler than the other
     two options, are safer to use with possibly-hostile pattern sources.
    </para>
   </caution>

   <para>
    <function>SIMILAR TO</function> and <acronym>POSIX</acronym>-style regular
    expressions do not support nondeterministic collations.  If required, use
    <function>LIKE</function> or apply a different collation to the expression
    to work around this limitation.
   </para>

  <sect2 id="functions-like">
   <title><function>LIKE</function></title>

   <indexterm>
    <primary>LIKE</primary>
   </indexterm>

<synopsis>
<replaceable>string</replaceable> LIKE <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
<replaceable>string</replaceable> NOT LIKE <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
</synopsis>

    <para>
     The <function>LIKE</function> expression returns true if the
     <replaceable>string</replaceable> matches the supplied
     <replaceable>pattern</replaceable>.  (As
     expected, the <function>NOT LIKE</function> expression returns
     false if <function>LIKE</function> returns true, and vice versa.
     An equivalent expression is
     <literal>NOT (<replaceable>string</replaceable> LIKE
      <replaceable>pattern</replaceable>)</literal>.)
    </para>

    <para>
     If <replaceable>pattern</replaceable> does not contain percent
     signs or underscores, then the pattern only represents the string
     itself; in that case <function>LIKE</function> acts like the
     equals operator.  An underscore (<literal>_</literal>) in
     <replaceable>pattern</replaceable> stands for (matches) any single
     character; a percent sign (<literal>%</literal>) matches any sequence
     of zero or more characters.
    </para>

   <para>
    Some examples:
<programlisting>
'abc' LIKE 'abc'    <lineannotation>true</lineannotation>
'abc' LIKE 'a%'     <lineannotation>true</lineannotation>
'abc' LIKE '_b_'    <lineannotation>true</lineannotation>
'abc' LIKE 'c'      <lineannotation>false</lineannotation>
</programlisting>
   </para>

   <para>
    <function>LIKE</function> pattern matching supports nondeterministic
    collations (see <xref linkend="collation-nondeterministic"/>), such as
    case-insensitive collations or collations that, say, ignore punctuation.
    So with a case-insensitive collation, one could have:
<programlisting>
'AbC' LIKE 'abc' COLLATE case_insensitive    <lineannotation>true</lineannotation>
'AbC' LIKE 'a%' COLLATE case_insensitive     <lineannotation>true</lineannotation>
</programlisting>
    With collations that ignore certain characters or in general that consider
    strings of different lengths equal, the semantics can become a bit more
    complicated.  Consider these examples:
<programlisting>
'.foo.' LIKE 'foo' COLLATE ign_punct    <lineannotation>true</lineannotation>
'.foo.' LIKE 'f_o' COLLATE ign_punct    <lineannotation>true</lineannotation>
'.foo.' LIKE '_oo' COLLATE ign_punct    <lineannotation>false</lineannotation>
</programlisting>
    The way the matching works is that the pattern is partitioned into
    sequences of wildcards and non-wildcard strings (wildcards being
    <literal>_</literal> and <literal>%</literal>).  For example, the pattern
    <literal>f_o</literal> is partitioned into <literal>f, _, o</literal>, the
    pattern <literal>_oo</literal> is partitioned into <literal>_,
    oo</literal>.  The input string matches the pattern if it can be
    partitioned in such a way that the wildcards match one character or any
    number of characters respectively and the non-wildcard partitions are
    equal under the applicable collation.  So for example, <literal>'.foo.'
    LIKE 'f_o' COLLATE ign_punct</literal> is true because one can partition
    <literal>.foo.</literal> into <literal>.f, o, o.</literal>, and then
    <literal>'.f' = 'f' COLLATE ign_punct</literal>, <literal>'o'</literal>
    matches the <literal>_</literal> wildcard, and <literal>'o.' = 'o' COLLATE
    ign_punct</literal>.  But <literal>'.foo.' LIKE '_oo' COLLATE
    ign_punct</literal> is false because <literal>.foo.</literal> cannot be
    partitioned in a way that the first character is any character and the
    rest of the string compares equal to <literal>oo</literal>.  (Note that
    the single-character wildcard always matches exactly one character,
    independent of the collation.  So in this example, the
    <literal>_</literal> would match <literal>.</literal>, but then the rest
    of the input string won't match the rest of the pattern.)
   </para>

   <para>
    <function>LIKE</function> pattern matching always covers the entire
    string.  Therefore, if it's desired to match a sequence anywhere within
    a string, the pattern must start and end with a percent sign.
   </para>

   <para>
    To match a literal underscore or percent sign without matching
    other characters, the respective character in
    <replaceable>pattern</replaceable> must be
    preceded by the escape character.  The default escape
    character is the backslash but a different one can be selected by
    using the <literal>ESCAPE</literal> clause.  To match the escape
    character itself, write two escape characters.
   </para>

   <para>
    It's also possible to select no escape character by writing
    <literal>ESCAPE ''</literal>.  This effectively disables the
    escape mechanism, which makes it impossible to turn off the
    special meaning of underscore and percent signs in the pattern.
   </para>

   <para>
    According to the SQL standard, omitting <literal>ESCAPE</literal>
    means there is no escape character (rather than defaulting to a
    backslash), and a zero-length <literal>ESCAPE</literal> value is
    disallowed.  <productname>PostgreSQL</productname>'s behavior in
    this regard is therefore slightly nonstandard.
   </para>

   <para>
    The key word <token>ILIKE</token> can be used instead of
    <token>LIKE</token> to make the match case-insensitive according to the
    active locale.  (But this does not support nondeterministic collations.)
    This is not in the <acronym>SQL</acronym> standard but is a
    <productname>PostgreSQL</productname> extension.
   </para>

   <para>
    The operator <literal>~~</literal> is equivalent to
    <function>LIKE</function>, and <literal>~~*</literal> corresponds to
    <function>ILIKE</function>.  There are also
    <literal>!~~</literal> and <literal>!~~*</literal> operators that
    represent <function>NOT LIKE</function> and <function>NOT
    ILIKE</function>, respectively.  All of these operators are
    <productname>PostgreSQL</productname>-specific.  You may see these
    operator names in <command>EXPLAIN</command> output and similar
    places, since the parser actually translates <function>LIKE</function>
    et al. to these operators.
   </para>

   <para>
    The phrases <function>LIKE</function>, <function>ILIKE</function>,
    <function>NOT LIKE</function>, and <function>NOT ILIKE</function> are
    generally treated as operators
    in <productname>PostgreSQL</productname> syntax; for example they can
    be used in <replaceable>expression</replaceable>
    <replaceable>operator</replaceable> ANY
    (<replaceable>subquery</replaceable>) constructs, although
    an <literal>ESCAPE</literal> clause cannot be included there.  In some
    obscure cases it may be necessary to use the underlying operator names
    instead.
   </para>

   <para>
    Also see the starts-with operator <literal>^@</literal> and the
    corresponding <function>starts_with()</function> function, which are
    useful in cases where simply matching the beginning of a string is
    needed.
   </para>
  </sect2>


  <sect2 id="functions-similarto-regexp">
   <title><function>SIMILAR TO</function> Regular Expressions</title>

   <indexterm>
    <primary>regular expression</primary>
    <!-- <seealso>pattern matching</seealso> breaks index build -->
   </indexterm>

   <indexterm>
    <primary>SIMILAR TO</primary>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>

<synopsis>
<replaceable>string</replaceable> SIMILAR TO <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
<replaceable>string</replaceable> NOT SIMILAR TO <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
</synopsis>

   <para>
    The <function>SIMILAR TO</function> operator returns true or
    false depending on whether its pattern matches the given string.
    It is similar to <function>LIKE</function>, except that it
    interprets the pattern using the SQL standard's definition of a
    regular expression.  SQL regular expressions are a curious cross
    between <function>LIKE</function> notation and common (POSIX) regular
    expression notation.
   </para>

   <para>
    Like <function>LIKE</function>, the <function>SIMILAR TO</function>
    operator succeeds only if its pattern matches the entire string;
    this is unlike common regular expression behavior where the pattern
    can match any part of the string.
    Also like
    <function>LIKE</function>, <function>SIMILAR TO</function> uses
    <literal>_</literal> and <literal>%</literal> as wildcard characters denoting
    any single character and any string, respectively (these are
    comparable to <literal>.</literal> and <literal>.*</literal> in POSIX regular
    expressions).
   </para>

   <para>
    In addition to these facilities borrowed from <function>LIKE</function>,
    <function>SIMILAR TO</function> supports these pattern-matching
    metacharacters borrowed from POSIX regular expressions:

   <itemizedlist>
    <listitem>
     <para>
      <literal>|</literal> denotes alternation (either of two alternatives).
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>*</literal> denotes repetition of the previous item zero
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>+</literal> denotes repetition of the previous item one
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>?</literal> denotes repetition of the previous item zero
      or one time.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>}</literal> denotes repetition
      of the previous item exactly <replaceable>m</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> denotes repetition
      of the previous item <replaceable>m</replaceable> or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal>
      denotes repetition of the previous item at least <replaceable>m</replaceable> and
      not more than <replaceable>n</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      Parentheses <literal>()</literal> can be used to group items into
      a single logical item.
     </para>
    </listitem>
    <listitem>
     <para>
      A bracket expression <literal>[...]</literal> specifies a character
      class, just as in POSIX regular expressions.
     </para>
    </listitem>
   </itemizedlist>

    Notice that the period (<literal>.</literal>) is not a metacharacter
    for <function>SIMILAR TO</function>.
   </para>

   <para>
    As with <function>LIKE</function>, a backslash disables the special
    meaning of any of these metacharacters.  A different escape character
    can be specified with <literal>ESCAPE</literal>, or the escape
    capability can be disabled by writing <literal>ESCAPE ''</literal>.
   </para>

   <para>
    According to the SQL standard, omitting <literal>ESCAPE</literal>
    means there is no escape character (rather than defaulting to a
    backslash), and a zero-length <literal>ESCAPE</literal> value is
    disallowed.  <productname>PostgreSQL</productname>'s behavior in
    this regard is therefore slightly nonstandard.
   </para>

   <para>
    Another nonstandard extension is that following the escape character
    with a letter or digit provides access to the escape sequences
    defined for POSIX regular expressions; see
    <xref linkend="posix-character-entry-escapes-table"/>,
    <xref linkend="posix-class-shorthand-escapes-table"/>, and
    <xref linkend="posix-constraint-escapes-table"/> below.
   </para>

   <para>
    Some examples:
<programlisting>
'abc' SIMILAR TO 'abc'          <lineannotation>true</lineannotation>
'abc' SIMILAR TO 'a'            <lineannotation>false</lineannotation>
'abc' SIMILAR TO '%(b|d)%'      <lineannotation>true</lineannotation>
'abc' SIMILAR TO '(b|c)%'       <lineannotation>false</lineannotation>
'-abc-' SIMILAR TO '%\mabc\M%'  <lineannotation>true</lineannotation>
'xabcy' SIMILAR TO '%\mabc\M%'  <lineannotation>false</lineannotation>
</programlisting>
   </para>

   <para>
    The <function>substring</function> function with three parameters
    provides extraction of a substring that matches an SQL
    regular expression pattern.  The function can be written according
    to standard SQL syntax:
<synopsis>
substring(<replaceable>string</replaceable> SIMILAR <replaceable>pattern</replaceable> ESCAPE <replaceable>escape-character</replaceable>)
</synopsis>
    or using the now obsolete SQL:1999 syntax:
<synopsis>
substring(<replaceable>string</replaceable> FROM <replaceable>pattern</replaceable> FOR <replaceable>escape-character</replaceable>)
</synopsis>
    or as a plain three-argument function:
<synopsis>
substring(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>, <replaceable>escape-character</replaceable>)
</synopsis>
    As with <literal>SIMILAR TO</literal>, the
    specified pattern must match the entire data string, or else the
    function fails and returns null.  To indicate the part of the
    pattern for which the matching data sub-string is of interest,
    the pattern should contain
    two occurrences of the escape character followed by a double quote
    (<literal>"</literal>). <!-- " font-lock sanity -->
    The text matching the portion of the pattern
    between these separators is returned when the match is successful.
   </para>

   <para>
    The escape-double-quote separators actually
    divide <function>substring</function>'s pattern into three independent
    regular expressions; for example, a vertical bar (<literal>|</literal>)
    in any of the three sections affects only that section.  Also, the first
    and third of these regular expressions are defined to match the smallest
    possible amount of text, not the largest, when there is any ambiguity
    about how much of the data string matches which pattern.  (In POSIX
    parlance, the first and third regular expressions are forced to be
    non-greedy.)
   </para>

   <para>
    As an extension to the SQL standard, <productname>PostgreSQL</productname>
    allows there to be just one escape-double-quote separator, in which case
    the third regular expression is taken as empty; or no separators, in which
    case the first and third regular expressions are taken as empty.
   </para>

   <para>
    Some examples, with <literal>#&quot;</literal> delimiting the return string:
<programlisting>
substring('foobar' SIMILAR '%#"o_b#"%' ESCAPE '#')   <lineannotation>oob</lineannotation>
substring('foobar' SIMILAR '#"o_b#"%' ESCAPE '#')    <lineannotation>NULL</lineannotation>
</programlisting>
   </para>
  </sect2>

  <sect2 id="functions-posix-regexp">
   <title><acronym>POSIX</acronym> Regular Expressions</title>

   <indexterm zone="functions-posix-regexp">
    <primary>regular expression</primary>
    <seealso>pattern matching</seealso>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_count</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_instr</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_like</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_match</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_matches</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_replace</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_table</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_array</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_substr</primary>
   </indexterm>

   <para>
    <xref linkend="functions-posix-table"/> lists the available
    operators for pattern matching using POSIX regular expressions.
   </para>

   <table id="functions-posix-table">
    <title>Regular Expression Match Operators</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' ~ 't.*ma'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case-insensitively
       </para>
       <para>
        <literal>'thomas' ~* 'T.*ma'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' !~ 't.*max'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case-insensitively
       </para>
       <para>
        <literal>'thomas' !~* 'T.*ma'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
     <acronym>POSIX</acronym> regular expressions provide a more
     powerful means for pattern matching than the <function>LIKE</function> and
     <function>SIMILAR TO</function> operators.
     Many Unix tools such as <command>egrep</command>,
     <command>sed</command>, or <command>awk</command> use a pattern
     matching language that is similar to the one described here.
    </para>

    <para>
     A regular expression is a character sequence that is an
     abbreviated definition of a set of strings (a <firstterm>regular
     set</firstterm>).  A string is said to match a regular expression
     if it is a member of the regular set described by the regular
     expression.  As with <function>LIKE</function>, pattern characters
     match string characters exactly unless they are special characters
     in the regular expression language &mdash; but regular expressions use
     different special characters than <function>LIKE</function> does.
     Unlike <function>LIKE</function> patterns, a
     regular expression is allowed to match anywhere within a string, unless
     the regular expression is explicitly anchored to the beginning or
     end of the string.
    </para>

    <para>
     Some examples:
<programlisting>
'abcd' ~ 'bc'     <lineannotation>true</lineannotation>
'abcd' ~ 'a.c'    <lineannotation>true &mdash; dot matches any character</lineannotation>
'abcd' ~ 'a.*d'   <lineannotation>true &mdash; <literal>*</literal> repeats the preceding pattern item</lineannotation>
'abcd' ~ '(b|x)'  <lineannotation>true &mdash; <literal>|</literal> means OR, parentheses group</lineannotation>
'abcd' ~ '^a'     <lineannotation>true &mdash; <literal>^</literal> anchors to start of string</lineannotation>
'abcd' ~ '^(b|c)' <lineannotation>false &mdash; would match except for anchoring</lineannotation>
</programlisting>
    </para>

    <para>
     The <acronym>POSIX</acronym> pattern language is described in much
     greater detail below.
    </para>

    <para>
     The <function>substring</function> function with two parameters,
     <function>substring(<replaceable>string</replaceable> from
     <replaceable>pattern</replaceable>)</function>, provides extraction of a
     substring
     that matches a POSIX regular expression pattern.  It returns null if
     there is no match, otherwise the first portion of the text that matched the
     pattern.  But if the pattern contains any parentheses, the portion
     of the text that matched the first parenthesized subexpression (the
     one whose left parenthesis comes first) is
     returned.  You can put parentheses around the whole expression
     if you want to use parentheses within it without triggering this
     exception.  If you need parentheses in the pattern before the
     subexpression you want to extract, see the non-capturing parentheses
     described below.
    </para>

   <para>
    Some examples:
<programlisting>
substring('foobar' FROM 'o.b')     <lineannotation>oob</lineannotation>
substring('foobar' FROM 'o(.)b')   <lineannotation>o</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_count</function> function counts the number of
     places where a POSIX regular expression pattern matches a string.
     It has the syntax
     <function>regexp_count</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>
     <optional>, <replaceable>start</replaceable>
     <optional>, <replaceable>flags</replaceable>
     </optional></optional>).
     <replaceable>pattern</replaceable> is searched for
     in <replaceable>string</replaceable>, normally from the beginning of
     the string, but if the <replaceable>start</replaceable> parameter is
     provided then beginning from that character index.
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  For example, including <literal>i</literal> in
     <replaceable>flags</replaceable> specifies case-insensitive matching.
     Supported flags are described in
     <xref linkend="posix-embedded-options-table"/>.
    </para>

    <para>
     Some examples:
<programlisting>
regexp_count('ABCABCAXYaxy', 'A.')          <lineannotation>3</lineannotation>
regexp_count('ABCABCAXYaxy', 'A.', 1, 'i')  <lineannotation>4</lineannotation>
</programlisting>
    </para>

    <para>
     The <function>regexp_instr</function> function returns the starting or
     ending position of the <replaceable>N</replaceable>'th match of a
     POSIX regular expression pattern to a string, or zero if there is no
     such match.  It has the syntax
     <function>regexp_instr</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>
     <optional>, <replaceable>start</replaceable>
     <optional>, <replaceable>N</replaceable>
     <optional>, <replaceable>endoption</replaceable>
     <optional>, <replaceable>flags</replaceable>
     <optional>, <replaceable>subexpr</replaceable>
     </optional></optional></optional></optional></optional>).
     <replaceable>pattern</replaceable> is searched for
     in <replaceable>string</replaceable>, normally from the beginning of
     the string, but if the <replaceable>start</replaceable> parameter is
     provided then beginning from that character index.
     If <replaceable>N</replaceable> is specified
     then the <replaceable>N</replaceable>'th match of the pattern
     is located, otherwise the first match is located.
     If the <replaceable>endoption</replaceable> parameter is omitted or
     specified as zero, the function returns the position of the first
     character of the match.  Otherwise, <replaceable>endoption</replaceable>
     must be one, and the function returns the position of the character
     following the match.
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Supported flags are described
     in <xref linkend="posix-embedded-options-table"/>.
     For a pattern containing parenthesized
     subexpressions, <replaceable>subexpr</replaceable> is an integer
     indicating which subexpression is of interest: the result identifies
     the position of the substring matching that subexpression.
     Subexpressions are numbered in the order of their leading parentheses.
     When <replaceable>subexpr</replaceable> is omitted or zero, the result
     identifies the position of the whole match regardless of
     parenthesized subexpressions.
    </para>

    <para>
     Some examples:
<programlisting>
regexp_instr('number of your street, town zip, FR', '[^,]+', 1, 2)
                                   <lineannotation>23</lineannotation>
regexp_instr(string=>'ABCDEFGHI', pattern=>'(c..)(...)', start=>1, "N"=>1, endoption=>0, flags=>'i', subexpr=>2)
                                   <lineannotation>6</lineannotation>
</programlisting>
    </para>

    <para>
     The <function>regexp_like</function> function checks whether a match
     of a POSIX regular expression pattern occurs within a string,
     returning boolean true or false.  It has the syntax
     <function>regexp_like</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Supported flags are described
     in <xref linkend="posix-embedded-options-table"/>.
     This function has the same results as the <literal>~</literal>
     operator if no flags are specified.  If only the <literal>i</literal>
     flag is specified, it has the same results as
     the <literal>~*</literal> operator.
    </para>

    <para>
     Some examples:
<programlisting>
regexp_like('Hello World', 'world')       <lineannotation>false</lineannotation>
regexp_like('Hello World', 'world', 'i')  <lineannotation>true</lineannotation>
</programlisting>
    </para>

    <para>
     The <function>regexp_match</function> function returns a text array of
     matching substring(s) within the first match of a POSIX
     regular expression pattern to a string.  It has the syntax
     <function>regexp_match</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable> <optional>, <replaceable>flags</replaceable> </optional>).
     If there is no match, the result is <literal>NULL</literal>.
     If a match is found, and the <replaceable>pattern</replaceable> contains no
     parenthesized subexpressions, then the result is a single-element text
     array containing the substring matching the whole pattern.
     If a match is found, and the <replaceable>pattern</replaceable> contains
     parenthesized subexpressions, then the result is a text array
     whose <replaceable>n</replaceable>'th element is the substring matching
     the <replaceable>n</replaceable>'th parenthesized subexpression of
     the <replaceable>pattern</replaceable> (not counting <quote>non-capturing</quote>
     parentheses; see below for details).
     The <replaceable>flags</replaceable> parameter is an optional text string
     containing zero or more single-letter flags that change the function's
     behavior.  Supported flags are described
     in <xref linkend="posix-embedded-options-table"/>.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT regexp_match('foobarbequebaz', 'bar.*que');
 regexp_match
--------------
 {barbeque}
(1 row)

SELECT regexp_match('foobarbequebaz', '(bar)(beque)');
 regexp_match
--------------
 {bar,beque}
(1 row)
</programlisting>
   </para>

    <tip>
     <para>
      In the common case where you just want the whole matching substring
      or <literal>NULL</literal> for no match, the best solution is to
      use <function>regexp_substr()</function>.
      However, <function>regexp_substr()</function> only exists
      in <productname>PostgreSQL</productname> version 15 and up.  When
      working in older versions, you can extract the first element
      of <function>regexp_match()</function>'s result, for example:
<programlisting>
SELECT (regexp_match('foobarbequebaz', 'bar.*que'))[1];
 regexp_match
--------------
 barbeque
(1 row)
</programlisting>
     </para>
    </tip>

    <para>
     The <function>regexp_matches</function> function returns a set of text arrays
     of matching substring(s) within matches of a POSIX regular
     expression pattern to a string.  It has the same syntax as
     <function>regexp_match</function>.
     This function returns no rows if there is no match, one row if there is
     a match and the <literal>g</literal> flag is not given, or <replaceable>N</replaceable>
     rows if there are <replaceable>N</replaceable> matches and the <literal>g</literal> flag
     is given.  Each returned row is a text array containing the whole
     matched substring or the substrings matching parenthesized
     subexpressions of the <replaceable>pattern</replaceable>, just as described above
     for <function>regexp_match</function>.
     <function>regexp_matches</function> accepts all the flags shown
     in <xref linkend="posix-embedded-options-table"/>, plus
     the <literal>g</literal> flag which commands it to return all matches, not
     just the first one.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT regexp_matches('foo', 'not there');
 regexp_matches
----------------
(0 rows)

SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
 regexp_matches
----------------
 {bar,beque}
 {bazil,barf}
(2 rows)
</programlisting>
   </para>

   <tip>
    <para>
     In most cases <function>regexp_matches()</function> should be used with
     the <literal>g</literal> flag, since if you only want the first match, it's
     easier and more efficient to use <function>regexp_match()</function>.
     However, <function>regexp_match()</function> only exists
     in <productname>PostgreSQL</productname> version 10 and up.  When working in older
     versions, a common trick is to place a <function>regexp_matches()</function>
     call in a sub-select, for example:
<programlisting>
SELECT col1, (SELECT regexp_matches(col2, '(bar)(beque)')) FROM tab;
</programlisting>
     This produces a text array if there's a match, or <literal>NULL</literal> if
     not, the same as <function>regexp_match()</function> would do.  Without the
     sub-select, this query would produce no output at all for table rows
     without a match, which is typically not the desired behavior.
    </para>
   </tip>

    <para>
     The <function>regexp_replace</function> function provides substitution of
     new text for substrings that match POSIX regular expression patterns.
     It has the syntax
     <function>regexp_replace</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>, <replaceable>replacement</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>)
     or
     <function>regexp_replace</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>, <replaceable>replacement</replaceable>,
     <replaceable>start</replaceable>
     <optional>, <replaceable>N</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional></optional>).
     The source <replaceable>string</replaceable> is returned unchanged if
     there is no match to the <replaceable>pattern</replaceable>.  If there is a
     match, the <replaceable>string</replaceable> is returned with the
     <replaceable>replacement</replaceable> string substituted for the matching
     substring.  The <replaceable>replacement</replaceable> string can contain
     <literal>\</literal><replaceable>n</replaceable>, where <replaceable>n</replaceable> is 1
     through 9, to indicate that the source substring matching the
     <replaceable>n</replaceable>'th parenthesized subexpression of the pattern should be
     inserted, and it can contain <literal>\&amp;</literal> to indicate that the
     substring matching the entire pattern should be inserted.  Write
     <literal>\\</literal> if you need to put a literal backslash in the replacement
     text.
     <replaceable>pattern</replaceable> is searched for
     in <replaceable>string</replaceable>, normally from the beginning of
     the string, but if the <replaceable>start</replaceable> parameter is
     provided then beginning from that character index.
     By default, only the first match of the pattern is replaced.
     If <replaceable>N</replaceable> is specified and is greater than zero,
     then the <replaceable>N</replaceable>'th match of the pattern
     is replaced.
     If the <literal>g</literal> flag is given, or
     if <replaceable>N</replaceable> is specified and is zero, then all
     matches at or after the <replaceable>start</replaceable> position are
     replaced.  (The <literal>g</literal> flag is ignored
     when <replaceable>N</replaceable> is specified.)
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Supported flags (though
     not <literal>g</literal>) are
     described in <xref linkend="posix-embedded-options-table"/>.
    </para>

   <para>
    Some examples:
<programlisting>
regexp_replace('foobarbaz', 'b..', 'X')
                                   <lineannotation>fooXbaz</lineannotation>
regexp_replace('foobarbaz', 'b..', 'X', 'g')
                                   <lineannotation>fooXX</lineannotation>
regexp_replace('foobarbaz', 'b(..)', 'X\1Y', 'g')
                                   <lineannotation>fooXarYXazY</lineannotation>
regexp_replace('A PostgreSQL function', 'a|e|i|o|u', 'X', 1, 0, 'i')
                                   <lineannotation>X PXstgrXSQL fXnctXXn</lineannotation>
regexp_replace(string=>'A PostgreSQL function', pattern=>'a|e|i|o|u', replacement=>'X', start=>1, "N"=>3, flags=>'i')
                                   <lineannotation>A PostgrXSQL function</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_split_to_table</function> function splits a string using a POSIX
     regular expression pattern as a delimiter.  It has the syntax
     <function>regexp_split_to_table</function>(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     If there is no match to the <replaceable>pattern</replaceable>, the function returns the
     <replaceable>string</replaceable>.  If there is at least one match, for each match it returns
     the text from the end of the last match (or the beginning of the string)
     to the beginning of the match.  When there are no more matches, it
     returns the text from the end of the last match to the end of the string.
     The <replaceable>flags</replaceable> parameter is an optional text string containing
     zero or more single-letter flags that change the function's behavior.
     <function>regexp_split_to_table</function> supports the flags described in
     <xref linkend="posix-embedded-options-table"/>.
    </para>

    <para>
     The <function>regexp_split_to_array</function> function behaves the same as
     <function>regexp_split_to_table</function>, except that <function>regexp_split_to_array</function>
     returns its result as an array of <type>text</type>.  It has the syntax
     <function>regexp_split_to_array</function>(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     The parameters are the same as for <function>regexp_split_to_table</function>.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT foo FROM regexp_split_to_table('the quick brown fox jumps over the lazy dog', '\s+') AS foo;
  foo
-------
 the
 quick
 brown
 fox
 jumps
 over
 the
 lazy
 dog
(9 rows)

SELECT regexp_split_to_array('the quick brown fox jumps over the lazy dog', '\s+');
              regexp_split_to_array
-----------------------------------------------
 {the,quick,brown,fox,jumps,over,the,lazy,dog}
(1 row)

SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
 foo
-----
 t
 h
 e
 q
 u
 i
 c
 k
 b
 r
 o
 w
 n
 f
 o
 x
(16 rows)
</programlisting>
   </para>

   <para>
    As the last example demonstrates, the regexp split functions ignore
    zero-length matches that occur at the start or end of the string
    or immediately after a previous match.  This is contrary to the strict
    definition of regexp matching that is implemented by
    the other regexp functions, but is usually the most convenient behavior
    in practice.  Other software systems such as Perl use similar definitions.
   </para>

    <para>
     The <function>regexp_substr</function> function returns the substring
     that matches a POSIX regular expression pattern,
     or <literal>NULL</literal> if there is no match.  It has the syntax
     <function>regexp_substr</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>
     <optional>, <replaceable>start</replaceable>
     <optional>, <replaceable>N</replaceable>
     <optional>, <replaceable>flags</replaceable>
     <optional>, <replaceable>subexpr</replaceable>
     </optional></optional></optional></optional>).
     <replaceable>pattern</replaceable> is searched for
     in <replaceable>string</replaceable>, normally from the beginning of
     the string, but if the <replaceable>start</replaceable> parameter is
     provided then beginning from that character index.
     If <replaceable>N</replaceable> is specified
     then the <replaceable>N</replaceable>'th match of the pattern
     is returned, otherwise the first match is returned.
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Supported flags are described
     in <xref linkend="posix-embedded-options-table"/>.
     For a pattern containing parenthesized
     subexpressions, <replaceable>subexpr</replaceable> is an integer
     indicating which subexpression is of interest: the result is the
     substring matching that subexpression.
     Subexpressions are numbered in the order of their leading parentheses.
     When <replaceable>subexpr</replaceable> is omitted or zero, the result
     is the whole match regardless of parenthesized subexpressions.
    </para>

    <para>
     Some examples:
<programlisting>
regexp_substr('number of your street, town zip, FR', '[^,]+', 1, 2)
                                   <lineannotation> town zip</lineannotation>
regexp_substr('ABCDEFGHI', '(c..)(...)', 1, 1, 'i', 2)
                                   <lineannotation>FGH</lineannotation>
</programlisting>
    </para>

<!-- derived from the re_syntax.n man page -->

   <sect3 id="posix-syntax-details">
    <title>Regular Expression Details</title>

   <para>
    <productname>PostgreSQL</productname>'s regular expressions are implemented
    using a software package written by Henry Spencer.  Much of
    the description of regular expressions below is copied verbatim from his
    manual.
   </para>

   <para>
    Regular expressions (<acronym>RE</acronym>s), as defined in
    <acronym>POSIX</acronym> 1003.2, come in two forms:
    <firstterm>extended</firstterm> <acronym>RE</acronym>s or <acronym>ERE</acronym>s
    (roughly those of <command>egrep</command>), and
    <firstterm>basic</firstterm> <acronym>RE</acronym>s or <acronym>BRE</acronym>s
    (roughly those of <command>ed</command>).
    <productname>PostgreSQL</productname> supports both forms, and
    also implements some extensions
    that are not in the POSIX standard, but have become widely used
    due to their availability in programming languages such as Perl and Tcl.
    <acronym>RE</acronym>s using these non-POSIX extensions are called
    <firstterm>advanced</firstterm> <acronym>RE</acronym>s or <acronym>ARE</acronym>s
    in this documentation.  AREs are almost an exact superset of EREs,
    but BREs have several notational incompatibilities (as well as being
    much more limited).
    We first describe the ARE and ERE forms, noting features that apply
    only to AREs, and then describe how BREs differ.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> always initially presumes that a regular
     expression follows the ARE rules.  However, the more limited ERE or
     BRE rules can be chosen by prepending an <firstterm>embedded option</firstterm>
     to the RE pattern, as described in <xref linkend="posix-metasyntax"/>.
     This can be useful for compatibility with applications that expect
     exactly the <acronym>POSIX</acronym> 1003.2 rules.
    </para>
   </note>

   <para>
    A regular expression is defined as one or more
    <firstterm>branches</firstterm>, separated by
    <literal>|</literal>.  It matches anything that matches one of the
    branches.
   </para>

   <para>
    A branch is zero or more <firstterm>quantified atoms</firstterm> or
    <firstterm>constraints</firstterm>, concatenated.
    It matches a match for the first, followed by a match for the second, etc.;
    an empty branch matches the empty string.
   </para>

   <para>
    A quantified atom is an <firstterm>atom</firstterm> possibly followed
    by a single <firstterm>quantifier</firstterm>.
    Without a quantifier, it matches a match for the atom.
    With a quantifier, it can match some number of matches of the atom.
    An <firstterm>atom</firstterm> can be any of the possibilities
    shown in <xref linkend="posix-atoms-table"/>.
    The possible quantifiers and their meanings are shown in
    <xref linkend="posix-quantifiers-table"/>.
   </para>

   <para>
    A <firstterm>constraint</firstterm> matches an empty string, but matches only when
    specific conditions are met.  A constraint can be used where an atom
    could be used, except it cannot be followed by a quantifier.
    The simple constraints are shown in
    <xref linkend="posix-constraints-table"/>;
    some more constraints are described later.
   </para>


   <table id="posix-atoms-table">
    <title>Regular Expression Atoms</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Atom</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>(</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> (where <replaceable>re</replaceable> is any regular expression)
       matches a match for
       <replaceable>re</replaceable>, with the match noted for possible reporting </entry>
       </row>

       <row>
       <entry> <literal>(?:</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> as above, but the match is not noted for reporting
       (a <quote>non-capturing</quote> set of parentheses)
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>.</literal> </entry>
       <entry> matches any single character </entry>
       </row>

       <row>
       <entry> <literal>[</literal><replaceable>chars</replaceable><literal>]</literal> </entry>
       <entry> a <firstterm>bracket expression</firstterm>,
       matching any one of the <replaceable>chars</replaceable> (see
       <xref linkend="posix-bracket-expressions"/> for more detail) </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>k</replaceable> </entry>
       <entry> (where <replaceable>k</replaceable> is a non-alphanumeric character)
       matches that character taken as an ordinary character,
       e.g., <literal>\\</literal> matches a backslash character </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>c</replaceable> </entry>
       <entry> where <replaceable>c</replaceable> is alphanumeric
       (possibly followed by other characters)
       is an <firstterm>escape</firstterm>, see <xref linkend="posix-escape-sequences"/>
       (AREs only; in EREs and BREs, this matches <replaceable>c</replaceable>) </entry>
       </row>

       <row>
       <entry> <literal>{</literal> </entry>
       <entry> when followed by a character other than a digit,
       matches the left-brace character <literal>{</literal>;
       when followed by a digit, it is the beginning of a
       <replaceable>bound</replaceable> (see below) </entry>
       </row>

       <row>
       <entry> <replaceable>x</replaceable> </entry>
       <entry> where <replaceable>x</replaceable> is a single character with no other
       significance, matches that character </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    An RE cannot end with a backslash (<literal>\</literal>).
   </para>

   <table id="posix-quantifiers-table">
    <title>Regular Expression Quantifiers</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Quantifier</entry>
       <entry>Matches</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>*</literal> </entry>
       <entry> a sequence of 0 or more matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>+</literal> </entry>
       <entry> a sequence of 1 or more matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>?</literal> </entry>
       <entry> a sequence of 0 or 1 matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>}</literal> </entry>
       <entry> a sequence of exactly <replaceable>m</replaceable> matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> </entry>
       <entry> a sequence of <replaceable>m</replaceable> or more matches of the atom </entry>
       </row>

       <row>
       <entry>
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal> </entry>
       <entry> a sequence of <replaceable>m</replaceable> through <replaceable>n</replaceable>
       (inclusive) matches of the atom; <replaceable>m</replaceable> cannot exceed
       <replaceable>n</replaceable> </entry>
       </row>

       <row>
       <entry> <literal>*?</literal> </entry>
       <entry> non-greedy version of <literal>*</literal> </entry>
       </row>

       <row>
       <entry> <literal>+?</literal> </entry>
       <entry> non-greedy version of <literal>+</literal> </entry>
       </row>

       <row>
       <entry> <literal>??</literal> </entry>
       <entry> non-greedy version of <literal>?</literal> </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>}</literal> </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>,}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> </entry>
       </row>

       <row>
       <entry>
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The forms using <literal>{</literal><replaceable>...</replaceable><literal>}</literal>
    are known as <firstterm>bounds</firstterm>.
    The numbers <replaceable>m</replaceable> and <replaceable>n</replaceable> within a bound are
    unsigned decimal integers with permissible values from 0 to 255 inclusive.
   </para>

    <para>
     <firstterm>Non-greedy</firstterm> quantifiers (available in AREs only) match the
     same possibilities as their corresponding normal (<firstterm>greedy</firstterm>)
     counterparts, but prefer the smallest number rather than the largest
     number of matches.
     See <xref linkend="posix-matching-rules"/> for more detail.
   </para>

   <note>
    <para>
     A quantifier cannot immediately follow another quantifier, e.g.,
     <literal>**</literal> is invalid.
     A quantifier cannot
     begin an expression or subexpression or follow
     <literal>^</literal> or <literal>|</literal>.
    </para>
   </note>

   <table id="posix-constraints-table">
    <title>Regular Expression Constraints</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Constraint</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>^</literal> </entry>
       <entry> matches at the beginning of the string </entry>
       </row>

       <row>
       <entry> <literal>$</literal> </entry>
       <entry> matches at the end of the string </entry>
       </row>

       <row>
       <entry> <literal>(?=</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>positive lookahead</firstterm> matches at any point
       where a substring matching <replaceable>re</replaceable> begins
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?!</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>negative lookahead</firstterm> matches at any point
       where no substring matching <replaceable>re</replaceable> begins
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?&lt;=</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>positive lookbehind</firstterm> matches at any point
       where a substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?&lt;!</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>negative lookbehind</firstterm> matches at any point
       where no substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Lookahead and lookbehind constraints cannot contain <firstterm>back
    references</firstterm> (see <xref linkend="posix-escape-sequences"/>),
    and all parentheses within them are considered non-capturing.
   </para>
   </sect3>

   <sect3 id="posix-bracket-expressions">
    <title>Bracket Expressions</title>

   <para>
    A <firstterm>bracket expression</firstterm> is a list of
    characters enclosed in <literal>[]</literal>.  It normally matches
    any single character from the list (but see below).  If the list
    begins with <literal>^</literal>, it matches any single character
    <emphasis>not</emphasis> from the rest of the list.
    If two characters
    in the list are separated by <literal>-</literal>, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g., <literal>[0-9]</literal> in <acronym>ASCII</acronym> matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.,  <literal>a-c-e</literal>.  Ranges are very
    collating-sequence-dependent, so portable programs should avoid
    relying on them.
   </para>

   <para>
    To include a literal <literal>]</literal> in the list, make it the
    first character (after <literal>^</literal>, if that is used).  To
    include a literal <literal>-</literal>, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    <literal>-</literal> as the first endpoint of a range, enclose it
    in <literal>[.</literal> and <literal>.]</literal> to make it a
    collating element (see below).  With the exception of these characters,
    some combinations using <literal>[</literal>
    (see next paragraphs), and escapes (AREs only), all other special
    characters lose their special significance within a bracket expression.
    In particular, <literal>\</literal> is not special when following
    ERE or BRE rules, though it is special (as introducing an escape)
    in AREs.
   </para>

   <para>
    Within a bracket expression, a collating element (a character, a
    multiple-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    <literal>[.</literal> and <literal>.]</literal> stands for the
    sequence of characters of that collating element.  The sequence is
    treated as a single element of the bracket expression's list.  This
    allows a bracket
    expression containing a multiple-character collating element to
    match more than one character, e.g., if the collating sequence
    includes a <literal>ch</literal> collating element, then the RE
    <literal>[[.ch.]]*c</literal> matches the first five characters of
    <literal>chchcc</literal>.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> currently does not support multi-character collating
     elements. This information describes possible future behavior.
    </para>
   </note>

   <para>
    Within a bracket expression, a collating element enclosed in
    <literal>[=</literal> and <literal>=]</literal> is an <firstterm>equivalence
    class</firstterm>, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were <literal>[.</literal> and
    <literal>.]</literal>.)  For example, if <literal>o</literal> and
    <literal>^</literal> are the members of an equivalence class, then
    <literal>[[=o=]]</literal>, <literal>[[=^=]]</literal>, and
    <literal>[o^]</literal> are all synonymous.  An equivalence class
    cannot be an endpoint of a range.
   </para>

   <para>
    Within a bracket expression, the name of a character class
    enclosed in <literal>[:</literal> and <literal>:]</literal> stands
    for the list of all characters belonging to that class.  A character
    class cannot be used as an endpoint of a range.
    The <acronym>POSIX</acronym> standard defines these character class
    names:
    <literal>alnum</literal> (letters and numeric digits),
    <literal>alpha</literal> (letters),
    <literal>blank</literal> (space and tab),
    <literal>cntrl</literal> (control characters),
    <literal>digit</literal> (numeric digits),
    <literal>graph</literal> (printable characters except space),
    <literal>lower</literal> (lower-case letters),
    <literal>print</literal> (printable characters including space),
    <literal>punct</literal> (punctuation),
    <literal>space</literal> (any white space),
    <literal>upper</literal> (upper-case letters),
    and <literal>xdigit</literal> (hexadecimal digits).
    The behavior of these standard character classes is generally
    consistent across platforms for characters in the 7-bit ASCII set.
    Whether a given non-ASCII character is considered to belong to one
    of these classes depends on the <firstterm>collation</firstterm>
    that is used for the regular-expression function or operator
    (see <xref linkend="collation"/>), or by default on the
    database's <envar>LC_CTYPE</envar> locale setting (see
    <xref linkend="locale"/>).  The classification of non-ASCII
    characters can vary across platforms even in similarly-named
    locales.  (But the <literal>C</literal> locale never considers any
    non-ASCII characters to belong to any of these classes.)
    In addition to these standard character
    classes, <productname>PostgreSQL</productname> defines
    the <literal>word</literal> character class, which is the same as
    <literal>alnum</literal> plus the underscore (<literal>_</literal>)
    character, and
    the <literal>ascii</literal> character class, which contains exactly
    the 7-bit ASCII set.
   </para>

   <para>
    There are two special cases of bracket expressions:  the bracket
    expressions <literal>[[:&lt;:]]</literal> and
    <literal>[[:&gt;:]]</literal> are constraints,
    matching empty strings at the beginning
    and end of a word respectively.  A word is defined as a sequence
    of word characters that is neither preceded nor followed by word
    characters.  A word character is any character belonging to the
    <literal>word</literal> character class, that is, any letter, digit,
    or underscore.  This is an extension, compatible with but not
    specified by <acronym>POSIX</acronym> 1003.2, and should be used with
    caution in software intended to be portable to other systems.
    The constraint escapes described below are usually preferable; they
    are no more standard, but are easier to type.
   </para>
   </sect3>

   <sect3 id="posix-escape-sequences">
    <title>Regular Expression Escapes</title>

   <para>
    <firstterm>Escapes</firstterm> are special sequences beginning with <literal>\</literal>
    followed by an alphanumeric character. Escapes come in several varieties:
    character entry, class shorthands, constraint escapes, and back references.
    A <literal>\</literal> followed by an alphanumeric character but not constituting
    a valid escape is illegal in AREs.
    In EREs, there are no escapes: outside a bracket expression,
    a <literal>\</literal> followed by an alphanumeric character merely stands for
    that character as an ordinary character, and inside a bracket expression,
    <literal>\</literal> is an ordinary character.
    (The latter is the one actual incompatibility between EREs and AREs.)
   </para>

   <para>
    <firstterm>Character-entry escapes</firstterm> exist to make it easier to specify
    non-printing and other inconvenient characters in REs.  They are
    shown in <xref linkend="posix-character-entry-escapes-table"/>.
   </para>

   <para>
    <firstterm>Class-shorthand escapes</firstterm> provide shorthands for certain
    commonly-used character classes.  They are
    shown in <xref linkend="posix-class-shorthand-escapes-table"/>.
   </para>

   <para>
    A <firstterm>constraint escape</firstterm> is a constraint,
    matching the empty string if specific conditions are met,
    written as an escape.  They are
    shown in <xref linkend="posix-constraint-escapes-table"/>.
   </para>

   <para>
    A <firstterm>back reference</firstterm> (<literal>\</literal><replaceable>n</replaceable>) matches the
    same string matched by the previous parenthesized subexpression specified
    by the number <replaceable>n</replaceable>
    (see <xref linkend="posix-constraint-backref-table"/>).  For example,
    <literal>([bc])\1</literal> matches <literal>bb</literal> or <literal>cc</literal>
    but not <literal>bc</literal> or <literal>cb</literal>.
    The subexpression must entirely precede the back reference in the RE.
    Subexpressions are numbered in the order of their leading parentheses.
    Non-capturing parentheses do not define subexpressions.
    The back reference considers only the string characters matched by the
    referenced subexpression, not any constraints contained in it.  For
    example, <literal>(^\d)\1</literal> will match <literal>22</literal>.
   </para>

   <table id="posix-character-entry-escapes-table">
    <title>Regular Expression Character-Entry Escapes</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\a</literal> </entry>
       <entry> alert (bell) character, as in C </entry>
       </row>

       <row>
       <entry> <literal>\b</literal> </entry>
       <entry> backspace, as in C </entry>
       </row>

       <row>
       <entry> <literal>\B</literal> </entry>
       <entry> synonym for backslash (<literal>\</literal>) to help reduce the need for backslash
       doubling </entry>
       </row>

       <row>
       <entry> <literal>\c</literal><replaceable>X</replaceable> </entry>
       <entry> (where <replaceable>X</replaceable> is any character) the character whose
       low-order 5 bits are the same as those of
       <replaceable>X</replaceable>, and whose other bits are all zero </entry>
       </row>

       <row>
       <entry> <literal>\e</literal> </entry>
       <entry> the character whose collating-sequence name
       is <literal>ESC</literal>,
       or failing that, the character with octal value <literal>033</literal> </entry>
       </row>

       <row>
       <entry> <literal>\f</literal> </entry>
       <entry> form feed, as in C </entry>
       </row>

       <row>
       <entry> <literal>\n</literal> </entry>
       <entry> newline, as in C </entry>
       </row>

       <row>
       <entry> <literal>\r</literal> </entry>
       <entry> carriage return, as in C </entry>
       </row>

       <row>
       <entry> <literal>\t</literal> </entry>
       <entry> horizontal tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\u</literal><replaceable>wxyz</replaceable> </entry>
       <entry> (where <replaceable>wxyz</replaceable> is exactly four hexadecimal digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>wxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\U</literal><replaceable>stuvwxyz</replaceable> </entry>
       <entry> (where <replaceable>stuvwxyz</replaceable> is exactly eight hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>stuvwxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\v</literal> </entry>
       <entry> vertical tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\x</literal><replaceable>hhh</replaceable> </entry>
       <entry> (where <replaceable>hhh</replaceable> is any sequence of hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>hhh</replaceable>
       (a single character no matter how many hexadecimal digits are used)
       </entry>
       </row>

       <row>
       <entry> <literal>\0</literal> </entry>
       <entry> the character whose value is <literal>0</literal> (the null byte)</entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>xy</replaceable> </entry>
       <entry> (where <replaceable>xy</replaceable> is exactly two octal digits,
       and is not a <firstterm>back reference</firstterm>)
       the character whose octal value is
       <literal>0</literal><replaceable>xy</replaceable> </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>xyz</replaceable> </entry>
       <entry> (where <replaceable>xyz</replaceable> is exactly three octal digits,
       and is not a <firstterm>back reference</firstterm>)
       the character whose octal value is
       <literal>0</literal><replaceable>xyz</replaceable> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Hexadecimal digits are <literal>0</literal>-<literal>9</literal>,
    <literal>a</literal>-<literal>f</literal>, and <literal>A</literal>-<literal>F</literal>.
    Octal digits are <literal>0</literal>-<literal>7</literal>.
   </para>

   <para>
    Numeric character-entry escapes specifying values outside the ASCII range
    (0&ndash;127) have meanings dependent on the database encoding.  When the
    encoding is UTF-8, escape values are equivalent to Unicode code points,
    for example <literal>\u1234</literal> means the character <literal>U+1234</literal>.
    For other multibyte encodings, character-entry escapes usually just
    specify the concatenation of the byte values for the character.  If the
    escape value does not correspond to any legal character in the database
    encoding, no error will be raised, but it will never match any data.
   </para>

   <para>
    The character-entry escapes are always taken as ordinary characters.
    For example, <literal>\135</literal> is <literal>]</literal> in ASCII, but
    <literal>\135</literal> does not terminate a bracket expression.
   </para>

   <table id="posix-class-shorthand-escapes-table">
    <title>Regular Expression Class-Shorthand Escapes</title>

    <tgroup cols="2">
      <colspec colname="col1" colwidth="1*"/>
      <colspec colname="col2" colwidth="3*"/>
      <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\d</literal> </entry>
       <entry> matches any digit, like
        <literal>[[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\s</literal> </entry>
       <entry> matches any whitespace character, like
        <literal>[[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\w</literal> </entry>
       <entry> matches any word character, like
        <literal>[[:word:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\D</literal> </entry>
       <entry> matches any non-digit, like
        <literal>[^[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\S</literal> </entry>
       <entry> matches any non-whitespace character, like
        <literal>[^[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\W</literal> </entry>
       <entry> matches any non-word character, like
        <literal>[^[:word:]]</literal> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The class-shorthand escapes also work within bracket expressions,
    although the definitions shown above are not quite syntactically
    valid in that context.
    For example, <literal>[a-c\d]</literal> is equivalent to
    <literal>[a-c[:digit:]]</literal>.
   </para>

   <table id="posix-constraint-escapes-table">
    <title>Regular Expression Constraint Escapes</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\A</literal> </entry>
       <entry> matches only at the beginning of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>^</literal>) </entry>
       </row>

       <row>
       <entry> <literal>\m</literal> </entry>
       <entry> matches only at the beginning of a word </entry>
       </row>

       <row>
       <entry> <literal>\M</literal> </entry>
       <entry> matches only at the end of a word </entry>
       </row>

       <row>
       <entry> <literal>\y</literal> </entry>
       <entry> matches only at the beginning or end of a word </entry>
       </row>

       <row>
       <entry> <literal>\Y</literal> </entry>
       <entry> matches only at a point that is not the beginning or end of a
       word </entry>
       </row>

       <row>
       <entry> <literal>\Z</literal> </entry>
       <entry> matches only at the end of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>$</literal>) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    A word is defined as in the specification of
    <literal>[[:&lt;:]]</literal> and <literal>[[:&gt;:]]</literal> above.
    Constraint escapes are illegal within bracket expressions.
   </para>

   <table id="posix-constraint-backref-table">
    <title>Regular Expression Back References</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\</literal><replaceable>m</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit)
       a back reference to the <replaceable>m</replaceable>'th subexpression </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>mnn</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit, and
       <replaceable>nn</replaceable> is some more digits, and the decimal value
       <replaceable>mnn</replaceable> is not greater than the number of closing capturing
       parentheses seen so far)
       a back reference to the <replaceable>mnn</replaceable>'th subexpression </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     There is an inherent ambiguity between octal character-entry
     escapes and back references, which is resolved by the following heuristics,
     as hinted at above.
     A leading zero always indicates an octal escape.
     A single non-zero digit, not followed by another digit,
     is always taken as a back reference.
     A multi-digit sequence not starting with a zero is taken as a back
     reference if it comes after a suitable subexpression
     (i.e., the number is in the legal range for a back reference),
     and otherwise is taken as octal.
    </para>
   </note>
   </sect3>

   <sect3 id="posix-metasyntax">
    <title>Regular Expression Metasyntax</title>

   <para>
    In addition to the main syntax described above, there are some special
    forms and miscellaneous syntactic facilities available.
   </para>

   <para>
    An RE can begin with one of two special <firstterm>director</firstterm> prefixes.
    If an RE begins with <literal>***:</literal>,
    the rest of the RE is taken as an ARE.  (This normally has no effect in
    <productname>PostgreSQL</productname>, since REs are assumed to be AREs;
    but it does have an effect if ERE or BRE mode had been specified by
    the <replaceable>flags</replaceable> parameter to a regex function.)
    If an RE begins with <literal>***=</literal>,
    the rest of the RE is taken to be a literal string,
    with all characters considered ordinary characters.
   </para>

   <para>
    An ARE can begin with <firstterm>embedded options</firstterm>:
    a sequence <literal>(?</literal><replaceable>xyz</replaceable><literal>)</literal>
    (where <replaceable>xyz</replaceable> is one or more alphabetic characters)
    specifies options affecting the rest of the RE.
    These options override any previously determined options &mdash;
    in particular, they can override the case-sensitivity behavior implied by
    a regex operator, or the <replaceable>flags</replaceable> parameter to a regex
    function.
    The available option letters are
    shown in <xref linkend="posix-embedded-options-table"/>.
    Note that these same option letters are used in the <replaceable>flags</replaceable>
    parameters of regex functions.
   </para>

   <table id="posix-embedded-options-table">
    <title>ARE Embedded-Option Letters</title>

    <tgroup cols="2">
     <colspec colname="col1" colwidth="1*"/>
     <colspec colname="col2" colwidth="3*"/>
     <thead>
      <row>
       <entry>Option</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>b</literal> </entry>
       <entry> rest of RE is a BRE </entry>
       </row>

       <row>
       <entry> <literal>c</literal> </entry>
       <entry> case-sensitive matching (overrides operator type) </entry>
       </row>

       <row>
       <entry> <literal>e</literal> </entry>
       <entry> rest of RE is an ERE </entry>
       </row>

       <row>
       <entry> <literal>i</literal> </entry>
       <entry> case-insensitive matching (see
       <xref linkend="posix-matching-rules"/>) (overrides operator type) </entry>
       </row>

       <row>
       <entry> <literal>m</literal> </entry>
       <entry> historical synonym for <literal>n</literal> </entry>
       </row>

       <row>
       <entry> <literal>n</literal> </entry>
       <entry> newline-sensitive matching (see
       <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>p</literal> </entry>
       <entry> partial newline-sensitive matching (see
       <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>q</literal> </entry>
       <entry> rest of RE is a literal (<quote>quoted</quote>) string, all ordinary
       characters </entry>
       </row>

       <row>
       <entry> <literal>s</literal> </entry>
       <entry> non-newline-sensitive matching (default) </entry>
       </row>

       <row>
       <entry> <literal>t</literal> </entry>
       <entry> tight syntax (default; see below) </entry>
       </row>

       <row>
       <entry> <literal>w</literal> </entry>
       <entry> inverse partial newline-sensitive (<quote>weird</quote>) matching
       (see <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>x</literal> </entry>
       <entry> expanded syntax (see below) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Embedded options take effect at the <literal>)</literal> terminating the sequence.
    They can appear only at the start of an ARE (after the
    <literal>***:</literal> director if any).
   </para>

   <para>
    In addition to the usual (<firstterm>tight</firstterm>) RE syntax, in which all
    characters are significant, there is an <firstterm>expanded</firstterm> syntax,
    available by specifying the embedded <literal>x</literal> option.
    In the expanded syntax,
    white-space characters in the RE are ignored, as are
    all characters between a <literal>#</literal>
    and the following newline (or the end of the RE).  This
    permits paragraphing and commenting a complex RE.
    There are three exceptions to that basic rule:

    <itemizedlist>
     <listitem>
      <para>
       a white-space character or <literal>#</literal> preceded by <literal>\</literal> is
       retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space or <literal>#</literal> within a bracket expression is retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space and comments cannot appear within multi-character symbols,
       such as <literal>(?:</literal>
      </para>
     </listitem>
    </itemizedlist>

    For this purpose, white-space characters are blank, tab, newline, and
    any character that belongs to the <replaceable>space</replaceable> character class.
   </para>

   <para>
    Finally, in an ARE, outside bracket expressions, the sequence
    <literal>(?#</literal><replaceable>ttt</replaceable><literal>)</literal>
    (where <replaceable>ttt</replaceable> is any text not containing a <literal>)</literal>)
    is a comment, completely ignored.
    Again, this is not allowed between the characters of
    multi-character symbols, like <literal>(?:</literal>.
    Such comments are more a historical artifact than a useful facility,
    and their use is deprecated; use the expanded syntax instead.
   </para>

   <para>
    <emphasis>None</emphasis> of these metasyntax extensions is available if
    an initial <literal>***=</literal> director
    has specified that the user's input be treated as a literal string
    rather than as an RE.
   </para>
   </sect3>

   <sect3 id="posix-matching-rules">
    <title>Regular Expression Matching Rules</title>

   <para>
    In the event that an RE could match more than one substring of a given
    string, the RE matches the one starting earliest in the string.
    If the RE could match more than one substring starting at that point,
    either the longest possible match or the shortest possible match will
    be taken, depending on whether the RE is <firstterm>greedy</firstterm> or
    <firstterm>non-greedy</firstterm>.
   </para>

   <para>
    Whether an RE is greedy or not is determined by the following rules:
    <itemizedlist>
     <listitem>
      <para>
       Most atoms, and all constraints, have no greediness attribute (because
       they cannot match variable amounts of text anyway).
      </para>
     </listitem>
     <listitem>
      <para>
       Adding parentheses around an RE does not change its greediness.
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with a fixed-repetition quantifier
       (<literal>{</literal><replaceable>m</replaceable><literal>}</literal>
       or
       <literal>{</literal><replaceable>m</replaceable><literal>}?</literal>)
       has the same greediness (possibly none) as the atom itself.
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with other normal quantifiers (including
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal>
       with <replaceable>m</replaceable> equal to <replaceable>n</replaceable>)
       is greedy (prefers longest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with a non-greedy quantifier (including
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}?</literal>
       with <replaceable>m</replaceable> equal to <replaceable>n</replaceable>)
       is non-greedy (prefers shortest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A branch &mdash; that is, an RE that has no top-level
       <literal>|</literal> operator &mdash; has the same greediness as the first
       quantified atom in it that has a greediness attribute.
      </para>
     </listitem>
     <listitem>
      <para>
       An RE consisting of two or more branches connected by the
       <literal>|</literal> operator is always greedy.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The above rules associate greediness attributes not only with individual
    quantified atoms, but with branches and entire REs that contain quantified
    atoms.  What that means is that the matching is done in such a way that
    the branch, or whole RE, matches the longest or shortest possible
    substring <emphasis>as a whole</emphasis>.  Once the length of the entire match
    is determined, the part of it that matches any particular subexpression
    is determined on the basis of the greediness attribute of that
    subexpression, with subexpressions starting earlier in the RE taking
    priority over ones starting later.
   </para>

   <para>
    An example of what this means:
<screen>
SELECT SUBSTRING('XY1234Z', 'Y*([0-9]{1,3})');
<lineannotation>Result: </lineannotation><computeroutput>123</computeroutput>
SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
    In the first case, the RE as a whole is greedy because <literal>Y*</literal>
    is greedy.  It can match beginning at the <literal>Y</literal>, and it matches
    the longest possible string starting there, i.e., <literal>Y123</literal>.
    The output is the parenthesized part of that, or <literal>123</literal>.
    In the second case, the RE as a whole is non-greedy because <literal>Y*?</literal>
    is non-greedy.  It can match beginning at the <literal>Y</literal>, and it matches
    the shortest possible string starting there, i.e., <literal>Y1</literal>.
    The subexpression <literal>[0-9]{1,3}</literal> is greedy but it cannot change
    the decision as to the overall match length; so it is forced to match
    just <literal>1</literal>.
   </para>

   <para>
    In short, when an RE contains both greedy and non-greedy subexpressions,
    the total match length is either as long as possible or as short as
    possible, according to the attribute assigned to the whole RE.  The
    attributes assigned to the subexpressions only affect how much of that
    match they are allowed to <quote>eat</quote> relative to each other.
   </para>

   <para>
    The quantifiers <literal>{1,1}</literal> and <literal>{1,1}?</literal>
    can be used to force greediness or non-greediness, respectively,
    on a subexpression or a whole RE.
    This is useful when you need the whole RE to have a greediness attribute
    different from what's deduced from its elements.  As an example,
    suppose that we are trying to separate a string containing some digits
    into the digits and the parts before and after them.  We might try to
    do that like this:
<screen>
SELECT regexp_match('abc01234xyz', '(.*)(\d+)(.*)');
<lineannotation>Result: </lineannotation><computeroutput>{abc0123,4,xyz}</computeroutput>
</screen>
    That didn't work: the first <literal>.*</literal> is greedy so
    it <quote>eats</quote> as much as it can, leaving the <literal>\d+</literal> to
    match at the last possible place, the last digit.  We might try to fix
    that by making it non-greedy:
<screen>
SELECT regexp_match('abc01234xyz', '(.*?)(\d+)(.*)');
<lineannotation>Result: </lineannotation><computeroutput>{abc,0,""}</computeroutput>
</screen>
    That didn't work either, because now the RE as a whole is non-greedy
    and so it ends the overall match as soon as possible.  We can get what
    we want by forcing the RE as a whole to be greedy:
<screen>
SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
<lineannotation>Result: </lineannotation><computeroutput>{abc,01234,xyz}</computeroutput>
</screen>
    Controlling the RE's overall greediness separately from its components'
    greediness allows great flexibility in handling variable-length patterns.
   </para>

   <para>
    When deciding what is a longer or shorter match,
    match lengths are measured in characters, not collating elements.
    An empty string is considered longer than no match at all.
    For example:
    <literal>bb*</literal>
    matches the three middle characters of <literal>abbbc</literal>;
    <literal>(week|wee)(night|knights)</literal>
    matches all ten characters of <literal>weeknights</literal>;
    when <literal>(.*).*</literal>
    is matched against <literal>abc</literal> the parenthesized subexpression
    matches all three characters; and when
    <literal>(a*)*</literal> is matched against <literal>bc</literal>
    both the whole RE and the parenthesized
    subexpression match an empty string.
   </para>

   <para>
    If case-independent matching is specified,
    the effect is much as if all case distinctions had vanished from the
    alphabet.
    When an alphabetic that exists in multiple cases appears as an
    ordinary character outside a bracket expression, it is effectively
    transformed into a bracket expression containing both cases,
    e.g., <literal>x</literal> becomes <literal>[xX]</literal>.
    When it appears inside a bracket expression, all case counterparts
    of it are added to the bracket expression, e.g.,
    <literal>[x]</literal> becomes <literal>[xX]</literal>
    and <literal>[^x]</literal> becomes <literal>[^xX]</literal>.
   </para>

   <para>
    If newline-sensitive matching is specified, <literal>.</literal>
    and bracket expressions using <literal>^</literal>
    will never match the newline character
    (so that matches will not cross lines unless the RE
    explicitly includes a newline)
    and <literal>^</literal> and <literal>$</literal>
    will match the empty string after and before a newline
    respectively, in addition to matching at beginning and end of string
    respectively.
    But the ARE escapes <literal>\A</literal> and <literal>\Z</literal>
    continue to match beginning or end of string <emphasis>only</emphasis>.
    Also, the character class shorthands <literal>\D</literal>
    and <literal>\W</literal> will match a newline regardless of this mode.
    (Before <productname>PostgreSQL</productname> 14, they did not match
    newlines when in newline-sensitive mode.
    Write <literal>[^[:digit:]]</literal>
    or <literal>[^[:word:]]</literal> to get the old behavior.)
   </para>

   <para>
    If partial newline-sensitive matching is specified,
    this affects <literal>.</literal> and bracket expressions
    as with newline-sensitive matching, but not <literal>^</literal>
    and <literal>$</literal>.
   </para>

   <para>
    If inverse partial newline-sensitive matching is specified,
    this affects <literal>^</literal> and <literal>$</literal>
    as with newline-sensitive matching, but not <literal>.</literal>
    and bracket expressions.
    This isn't very useful but is provided for symmetry.
   </para>
   </sect3>

   <sect3 id="posix-limits-compatibility">
    <title>Limits and Compatibility</title>

   <para>
    No particular limit is imposed on the length of REs in this
    implementation.  However,
    programs intended to be highly portable should not employ REs longer
    than 256 bytes,
    as a POSIX-compliant implementation can refuse to accept such REs.
   </para>

   <para>
    The only feature of AREs that is actually incompatible with
    POSIX EREs is that <literal>\</literal> does not lose its special
    significance inside bracket expressions.
    All other ARE features use syntax which is illegal or has
    undefined or unspecified effects in POSIX EREs;
    the <literal>***</literal> syntax of directors likewise is outside the POSIX
    syntax for both BREs and EREs.
   </para>

   <para>
    Many of the ARE extensions are borrowed from Perl, but some have
    been changed to clean them up, and a few Perl extensions are not present.
    Incompatibilities of note include <literal>\b</literal>, <literal>\B</literal>,
    the lack of special treatment for a trailing newline,
    the addition of complemented bracket expressions to the things
    affected by newline-sensitive matching,
    the restrictions on parentheses and back references in lookahead/lookbehind
    constraints, and the longest/shortest-match (rather than first-match)
    matching semantics.
   </para>
   </sect3>

   <sect3 id="posix-basic-regexes">
    <title>Basic Regular Expressions</title>

   <para>
    BREs differ from EREs in several respects.
    In BREs, <literal>|</literal>, <literal>+</literal>, and <literal>?</literal>
    are ordinary characters and there is no equivalent
    for their functionality.
    The delimiters for bounds are
    <literal>\{</literal> and <literal>\}</literal>,
    with <literal>{</literal> and <literal>}</literal>
    by themselves ordinary characters.
    The parentheses for nested subexpressions are
    <literal>\(</literal> and <literal>\)</literal>,
    with <literal>(</literal> and <literal>)</literal> by themselves ordinary characters.
    <literal>^</literal> is an ordinary character except at the beginning of the
    RE or the beginning of a parenthesized subexpression,
    <literal>$</literal> is an ordinary character except at the end of the
    RE or the end of a parenthesized subexpression,
    and <literal>*</literal> is an ordinary character if it appears at the beginning
    of the RE or the beginning of a parenthesized subexpression
    (after a possible leading <literal>^</literal>).
    Finally, single-digit back references are available, and
    <literal>\&lt;</literal> and <literal>\&gt;</literal>
    are synonyms for
    <literal>[[:&lt;:]]</literal> and <literal>[[:&gt;:]]</literal>
    respectively; no other escapes are available in BREs.
   </para>
   </sect3>

<!-- end re_syntax.n man page -->

   <sect3 id="posix-vs-xquery">
   <title>Differences from SQL Standard and XQuery</title>

   <indexterm zone="posix-vs-xquery">
    <primary>LIKE_REGEX</primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>OCCURRENCES_REGEX</primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>POSITION_REGEX</primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>SUBSTRING_REGEX</primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>TRANSLATE_REGEX</primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>XQuery regular expressions</primary>
   </indexterm>

    <para>
     Since SQL:2008, the SQL standard includes regular expression operators
     and functions that performs pattern
     matching according to the XQuery regular expression
     standard:
     <itemizedlist>
      <listitem><para><literal>LIKE_REGEX</literal></para></listitem>
      <listitem><para><literal>OCCURRENCES_REGEX</literal></para></listitem>
      <listitem><para><literal>POSITION_REGEX</literal></para></listitem>
      <listitem><para><literal>SUBSTRING_REGEX</literal></para></listitem>
      <listitem><para><literal>TRANSLATE_REGEX</literal></para></listitem>
     </itemizedlist>
     <productname>PostgreSQL</productname> does not currently implement these
     operators and functions.  You can get approximately equivalent
     functionality in each case as shown in <xref
     linkend="functions-regexp-sql-table"/>.  (Various optional clauses on
     both sides have been omitted in this table.)
    </para>

    <table id="functions-regexp-sql-table">
     <title>Regular Expression Functions Equivalencies</title>

     <tgroup cols="2">
      <thead>
       <row>
        <entry>SQL standard</entry>
        <entry><productname>PostgreSQL</productname></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal><replaceable>string</replaceable> LIKE_REGEX <replaceable>pattern</replaceable></literal></entry>
        <entry><literal>regexp_like(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal> or <literal><replaceable>string</replaceable> ~ <replaceable>pattern</replaceable></literal></entry>
       </row>

       <row>
        <entry><literal>OCCURRENCES_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable>)</literal></entry>
        <entry><literal>regexp_count(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
       </row>

       <row>
        <entry><literal>POSITION_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable>)</literal></entry>
        <entry><literal>regexp_instr(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
       </row>

       <row>
        <entry><literal>SUBSTRING_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable>)</literal></entry>
        <entry><literal>regexp_substr(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
       </row>

       <row>
        <entry><literal>TRANSLATE_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable> WITH <replaceable>replacement</replaceable>)</literal></entry>
        <entry><literal>regexp_replace(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>, <replaceable>replacement</replaceable>)</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
     Regular expression functions similar to those provided by PostgreSQL are
     also available in a number of other SQL implementations, whereas the
     SQL-standard functions are not as widely implemented.  Some of the
     details of the regular expression syntax will likely differ in each
     implementation.
    </para>

    <para>
     The SQL-standard operators and functions use XQuery regular expressions,
     which are quite close to the ARE syntax described above.
     Notable differences between the existing POSIX-based
     regular-expression feature and XQuery regular expressions include:

     <itemizedlist>
      <listitem>
       <para>
        XQuery character class subtraction is not supported.  An example of
        this feature is using the following to match only English
        consonants: <literal>[a-z-[aeiou]]</literal>.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery character class shorthands <literal>\c</literal>,
        <literal>\C</literal>, <literal>\i</literal>,
        and <literal>\I</literal> are not supported.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery character class elements
        using <literal>\p{UnicodeProperty}</literal> or the
        inverse <literal>\P{UnicodeProperty}</literal> are not supported.
       </para>
      </listitem>
      <listitem>
       <para>
        POSIX interprets character classes such as <literal>\w</literal>
        (see <xref linkend="posix-class-shorthand-escapes-table"/>)
        according to the prevailing locale (which you can control by
        attaching a <literal>COLLATE</literal> clause to the operator or
        function).  XQuery specifies these classes by reference to Unicode
        character properties, so equivalent behavior is obtained only with
        a locale that follows the Unicode rules.
       </para>
      </listitem>
      <listitem>
       <para>
        The SQL standard (not XQuery itself) attempts to cater for more
        variants of <quote>newline</quote> than POSIX does.  The
        newline-sensitive matching options described above consider only
        ASCII NL (<literal>\n</literal>) to be a newline, but SQL would have
        us treat CR (<literal>\r</literal>), CRLF (<literal>\r\n</literal>)
        (a Windows-style newline), and some Unicode-only characters like
        LINE SEPARATOR (U+2028) as newlines as well.
        Notably, <literal>.</literal> and <literal>\s</literal> should
        count <literal>\r\n</literal> as one character not two according to
        SQL.
       </para>
      </listitem>
      <listitem>
       <para>
        Of the character-entry escapes described in
        <xref linkend="posix-character-entry-escapes-table"/>,
        XQuery supports only <literal>\n</literal>, <literal>\r</literal>,
        and <literal>\t</literal>.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery does not support
        the <literal>[:<replaceable>name</replaceable>:]</literal> syntax
        for character classes within bracket expressions.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery does not have lookahead or lookbehind constraints,
        nor any of the constraint escapes described in
        <xref linkend="posix-constraint-escapes-table"/>.
       </para>
      </listitem>
      <listitem>
       <para>
        The metasyntax forms described in <xref linkend="posix-metasyntax"/>
        do not exist in XQuery.
       </para>
      </listitem>
      <listitem>
       <para>
        The regular expression flag letters defined by XQuery are
        related to but not the same as the option letters for POSIX
        (<xref linkend="posix-embedded-options-table"/>).  While the
        <literal>i</literal> and <literal>q</literal> options behave the
        same, others do not:
        <itemizedlist>
         <listitem>
          <para>
           XQuery's <literal>s</literal> (allow dot to match newline)
           and <literal>m</literal> (allow <literal>^</literal>
           and <literal>$</literal> to match at newlines) flags provide
           access to the same behaviors as
           POSIX's <literal>n</literal>, <literal>p</literal>
           and <literal>w</literal> flags, but they
           do <emphasis>not</emphasis> match the behavior of
           POSIX's <literal>s</literal> and <literal>m</literal> flags.
           Note in particular that dot-matches-newline is the default
           behavior in POSIX but not XQuery.
          </para>
         </listitem>
         <listitem>
          <para>
           XQuery's <literal>x</literal> (ignore whitespace in pattern) flag
           is noticeably different from POSIX's expanded-mode flag.
           POSIX's <literal>x</literal> flag also
           allows <literal>#</literal> to begin a comment in the pattern,
           and POSIX will not ignore a whitespace character after a
           backslash.
          </para>
         </listitem>
        </itemizedlist>
       </para>
      </listitem>
     </itemizedlist>
    </para>

   </sect3>
  </sect2>
 </sect1>
