Non-capturing parens ((?:...)) and flags ((?x-sim:...)) are used only in the parsing/compilation of the regular expression - they don't contribute nodes of their own to the resulting regexp program, but they may change what nodes something else compiles to.

For example, qr{.} gives the node 'SANY', while qr{.}s gives 'REG_ANY'.

However the compiler cannot stitch together subpatterns in compiled form, so whenever you build up a pattern from two previously qr'd patterns, or a qr'd pattern and some new text, perl constructs the stringified form of the new pattern and then compiles this new string from scratch.

Because of this, it is usually simpler for this sort of code to construct the string representing the complete pattern (with the added benefit that your diagnostics become more readable), so I'd tend to use:

  my @days    = qw( Sun Mon Tue Wed Thu Fri Sat );
  my $days_re = join '|', map "\Q$_\E", @days;
     $days_re = qr{$days_re};
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Hugo

However the compiler cannot stitch together subpatterns in compiled form, so whenever you build up a pattern from two previously qr'd patterns, or a qr'd pattern and some new text, perl constructs the stringified form of the new pattern and then compiles this new string from scratch.

Interesting! I wonder if there is a need for a low-level regex-manipulating package that would allow us to do manipulations like this on the actual regex objects. (Or if such an interface already exists!)

One comment / question on your preferred solution:

my $days_re = join '|', map "\Q$_\E", @days;
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I've seen this used a few times now, and I wonder if there is a meme out there people are copying from. Is there any reason to prefer this over:

my $days_re = join '|', map quotemeta, @days;
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I guess I think first of the \Q...\E form because I rarely quote things quite like this - more often I'm quoting a fragment inside a larger string.

Precisely because of that though, when reading the code you need to look more carefully at a \Q...\E construct, so quotemeta would have been the better thing to use in this case.

Note that this is a "belt and braces" approach: the quotemeta() isn't actually required for the strings specified, so this is a protection against future changes to the apparently constant data (and relatively unlikely changes as well, since I can't think of any languages that use regexp metacharacters in their names for the days of the week).

Hugo