How does the qr/STRING/ operator in Perl decide whether or not to compile STRING?

Question

The docs for qr/STRING/ say:

This operator quotes (and possibly compiles) its STRING as a regular expression.

What worries me is the part in parentheses. I can't think of any cases where I don't want it to compile a regex out of STRING. Is this parenthetical statement just weasel words to cover some future case where compiling is not desired or is there a case today (or in an earlier version of Perl) where STRING will not be compiled?

Answer 1

If the regex contains any interpolated strings, it always compiles (maybe not if you use the /o switch - I confess /o has always confused me). If the regex contains only literal text, I believe Adam's answer is correct.

IIRC it's meant to handle this scenario:

while (my $foo = $something->next) {
     my $regex1 = qr/ab(cd+)ef?/; # only compiled once
     my $regex2 = qr/ab${foo}*ef/; # compiled every time through the loop
     # do stuff with $regex1 and $regex2
}

Answer 2

Edit: This answer is wrong (or at least misguided) but there's some interesting discussion in the comments that's worth preserving. John Siracusa's answer appears to be on the right track.

---

The documentation for qr// states that

STRING is interpolated the same way as PATTERN in m/PATTERN/.

which presumably includes the behavior of not recompiling regular expressions for which the pattern hasn't changed, or can't change in the case of not including interpolated variables. For example, you don't need to recompile this pattern on each iteration of the loop:

foreach my $char ('a' .. 'z') {
    my $vowel = qr/[aeiou]/;
    say "$char is a vowel" if $char =~ $vowel;
}

Answer 3

The "possibly compiles" part of the documentation probably refers to situations like the one shown below, where the argument to qr// is an already-compiled regex:

use re 'debug';

$re1 = qr/foo/;
$re2 = qr/$re1/;

Running that program shows only one regex being compiled.

Regardless of the intent of that passage, sly allusions to internals details does not clear documentation make. I think a doc patch would be beneficial.