Beefy Boxes and Bandwidth Generously Provided by pair Networks
laziness, impatience, and hubris
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??
hv wrote:
Regexp union is not commutative when one of the alternates is a leading substring of another: then order becomes important - (E|x) will always match E in preference to x.
Excellent point. (However, union is commutative even in the face of overlapping operands when the union is forced to match all of the target string or not match at all.)
It is the presence of the outer anchors in the original pattern that disambiguates and thus makes (regexp union) commutative.
Even anchoring isn't sufficient. What if one of the anchors matches the difference between the union operands? Consider (a|aa) in:
"aaaa" =~ /a(a|aa)a/

Fortunately, in the case of the OP's regexes – s(?:al|pre)?ad and s(?:(?:al)?|pre)ad) – the union operands don't overlap, and so classical equivalence (do the regexps generate the same language?) predicts the equivalence of Perl's matching behavior (do the regexps match the same strings identically?)

Cheers,
Tom


In reply to Re^3: Why machine-generated solutions will never cease to amaze me by tmoertel
in thread Why machine-generated solutions will never cease to amaze me by grinder

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!
  • 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, 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, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            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.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others imbibing at the Monastery: (13)
    As of 2014-08-22 16:00 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The best computer themed movie is:











      Results (159 votes), past polls