controlling approximate matching parameters for the <i>atom</i>.
</p>


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<i>atom</i> ::= <b>"("</b> <i>extended-regexp</i> <b>")"</b>
     |   <a href="#bracket-expression"><i>bracket-expression</i></a>
     |   <b>"."</b>
     |   <a href="#assertion"><i>assertion</i></a>
     |   <a href="#literal"><i>literal</i></a>
     |   <a href="#backref"><i>back-reference</i></a>
     |   <b>"(?#"</b> <i>comment-text</i> <b>")"</b>
     |   <b>"(?"</b> <a hre