I have a couple places where I want to find /abc/ and replace b with d (actually they are more complex expressions, but this is the form it boils down to). So I added parens and wrote s/(a)b(c)/$1d$2/.

But I realized that this is what that "zero width lookbehind assertion" must be for. So, I'm going to review the perlre doc and learn how to use these extensions.

One reason I never bothered with them is that the docs are confusing.

But, to get started, am I correct in thinking that the equivilent would be s/(?<=a)b(?=c)/d/ ? And is that really any clearer?! What does "Works only for fixed-width look-behind" mean? Does that mean I can't have modifiers like + in there? In that case, what's the point, beyond saving a capture number in the cases where it does work?

—John

In reply to Regex replace in context by John M. Dlugosz

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.