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82th chunk of `doc/src/sgml/func.sgml`
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 <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">
     <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
Title: Regular Expression Bounds, Non-Greedy Quantifiers, Constraints, and Lookarounds
Summary
This section explains bounds in regular expressions, specifying that 'm' and 'n' within bounds must be unsigned decimal integers from 0 to 255. It introduces non-greedy quantifiers, which match the smallest possible number of occurrences. It also details regular expression constraints such as '^' (beginning of string), '$' (end of string), and lookahead/lookbehind assertions (available in AREs only), which match based on what follows or precedes the current position without consuming the matched characters. Lookahead and lookbehind constraints cannot contain back references, and parentheses within them are considered non-capturing.