'/turtle|fish/i' is a Perl expression, not a regular expression. turtle|fish and (?i-xsm:turtle|fish) are (Perl) regular expressions. Since '/turtle|fish/i' is Perl, you'd need eval to execute it. So you need to pass just the regular expression ((?i-xsm:turtle|fish)) or precompile it with qr// (qr/turtle|fish/i).

For example,

$uncompiled_re = '(?i-xsm:turtle|fish)';
mysub($uncomiled_re);

$compiled_re = qr/turtle|fish/i;
mysub($comiled_re);

[download]

Strings get tricky when \ is used, but qr// doesn't. For example,

$uncompiled_re = "(?i-xsm:\\w)";
mysub($uncomiled_re);

$compiled_re = qr/\w/i;
mysub($comiled_re);

[download]

The documented usage is
$string =~ /$re/
whether $re contains a string or a compiled regexp, but I think
$string =~ $re
works in both cases.

In reply to Re: Passing regular expressions as arguments to subroutines by ikegami
in thread Passing regular expressions as arguments to subroutines by Anonymous Monk

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