Re^3: simple pattern match ?

On Perl 5.8.5, the pattern works for me:

#!perl -w

for (qw(
  00 01 02 03 04 05 06 07 08 09
   0  1  2  3  4  5  6  7  8  9
  10 20 30 40 50 58 59
)) {
    die "Ooops: $_ didn't match the regular expression"
        unless /([012345]?\d)/;
};

print "All numbers passed.";
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Re^4: simple pattern match ? by McDarren (Abbot) on Mar 15, 2006 at 17:12 UTC
So it does :) So my logic above is flawed. I'm almost certain that I tested your pattern earlier and it failed - but obviously not. Anyway, I guess that means that the regex engine is "smart enough" to know that even if it gets a match with the `[012345]?` - it has to "release" it to the `\d` if there is no 2nd digit - yes?	[reply] [d/l] [select]
Re^5: simple pattern match ? by Corion (Patriarch) on Mar 15, 2006 at 17:15 UTC
Yes. The regex engine uses backtracking and marks its location whenever it has to make a decision. If the first decision doesn't work out, it backtracks to the last marked point and uses the next possible decision. The `?` regex operator is such a decision point and I guess that basically, the two possibilities for a match are: `[012345]\d` [download] and `\d` [download] So in this case, it would not get a match when trying `/[012345]\d/` against `5`, so it backtracks, and tries `/\d/`, which works.	[reply] [d/l] [select]