The Perl Regexp Engine of course. I have been working hard on working out how to formulate a regular expression with a negative lookbehind and I cannot get it right. What I have right now is this: /^(?<!XDG_) [_A-Z0-9]+ _PATH$/x

I'll explain in english what I need. There will be desired matches with strings like PKG_CONFIG_PATH but I must not match XDG_SEAT_PATH or XDG_SESSION_PATH. Naturally, it is not as simple as avoiding any match with a string beginning with XDG_ because I want to match XDG_DATA_DIRS, for example. (Yes, these are well-recognized environment variables).

This works for getting only PATH: /^(?<!XDG_)PATH$/, so I can see that the Engine does understand the lookbehind. Any suggestions will be appreciated!

Aug 22, 2025 at 13:49 UTC

A just machine to make big decisions
Programmed by fellows (and gals) with compassion and vision
We'll be clean when their work is done
We'll be eternally free yes, and eternally young
Donald Fagen —> I.G.Y.
(Slightly modified for inclusiveness)

In reply to Driving the Engine right off the road by Intrepid

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.