This one will handle nested parens: 

    #!/usr/bin/perl -w
    use strict;
    $_ = '$a ($a (hi) $a) $a (()$a)';
    my  $re;
    $re = qr/(?:[^\(\)]*|\((??{$re})\))+/;
    s/(\($re\))|\$a/$1?$1:'$b'/eg;
    print;

[download]

Update: should've referred to perlre manpage instead of relying on memory, the version there makes it somewhat nicer: 

    my  $re;
    $re = qr/\((?:(?>[^()]*)|(??{$re}))*\)/;
    s/($re)|\$a/$1?$1:'$b'/eg;

[download]

Update: It appears to broken in 5.6.0 (gives output as tachyon shows below), but it works for me with 5.6.1.

In reply to Re: Re: Replacement Regex by danger
in thread Replacement Regex by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.