Re^3: reg expression question

[Athanasius]

These special cases are fairly easy to accommodate:

1. To prevent 4 consecutive digits from being wrongly identified as a year, specify that the digits occur at the start of the right-hand string:

   if (my @m = $right =~ /^\d{2}(\d{2})(.*)/)
   #                      ^ Add this
   

   Within a regex, the special character ^ means “match at the start of the line.”

2. To remove spaces, use the substitution operator (with the /g modifier for global replacement):

   $right =~ s/\s+//g;
   

3. As for 2.

4. To prevent the string from ending in a dash, use the substitution operator again:

   $right =~ s/-$//;
   

   $ is another special regex character: it means “match at the end of the line.”

See “Metacharacters” in perlre.

Note the value of using a test-driven approach: I was able to add the 4 new input/output pairs to %data, make changes to get the new test cases to pass, and know that these modifications did not invalidate the original solution (because the 4 original test cases still pass).

Hope that helps,

[Anonymous Monk]

Within a regex, the special character ^ means “match at the start of the line”

Except when it means "match the beginning of the string" which is what yours means

Perl::Critic::Policy::RegularExpressions::RequireLineBoundaryMatching - Always use the /m modifier with regular expressions.

[Athanasius]

You are right, when used in a regex without an /m modifier, ^ means “match the beginning of the string,” and is equivalent to \A. “Match at the beginning of the line” (which is the definition given in perlre#Regular-Expressions) is strictly correct only when ^ is used in a regex with an /m modifier. So maybe this part of the documentation should be re-worded?

I haven’t used Perl::Critic, and as to this particular policy — well, I’m suspicious of guidelines that say “always do X” regardless of context. In the present case, adding an /m modifier to the regex would, IMO, be misleading, as it would imply (or at least suggest) that the string being matched is expected to contain multiple newlines. But I acknowledge that this is a judgment call, and YMMV.

[AnomalousMonk]

It is this "Without the /whatever modifier, whatever means ...; with the modifier it means..." locution that is the rationale for the kneejerk practice of always using the /xms constellation of modifiers with every regex regardless of whether  . ^ $ appear in the regex or not, and regardless of the fact that /x arguably makes it a bit more messy to deal with whitespace.

I have adopted this practice, first encountered in the (in)famous PBP of TheDamian, because for me, regexes are hard enough without the further confusion of unnecessary degrees of freedom. Now, what does  . (dot) match? Everything! YBPMV.

Update: See also re '/flags' mode, but I tend to avoid this in PM examples because it only appeared with Perl version 5.14 and has too narrow familiarity.