Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Let's say I wanted to pass a regex to a sub. How can I do this and validate the regex at the same time?

For example, sub('/turtle|fish/i') and then in the sub, I would have: 

sub {
# After validating that the regex is really a regex.
my $string = 'fish';
print "hello" if ($string =~ $_[0]); # prints hello
}

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Replies are listed 'Best First'.
Re: Passing regular expressions as arguments to subroutines
by johnnywang (Priest) on Nov 02, 2005 at 23:03 UTC
    Something like: 
    
my $a = qr/turtle|fish/i;
foo($a);

sub foo{
    my $reg = shift;
    if(ref($reg) eq 'Regexp'){
       my $string = 'fisH';
       print "hello" if ($string =~ $reg);
    }
}

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[d/l]
Re: Passing regular expressions as arguments to subroutines
by ikegami (Patriarch) on Nov 02, 2005 at 23:31 UTC

    '/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);

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    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.

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[d/l]
[select]
      The only reason the subroutine must only accept a regex is because that is the requirement imposed by the problem. Without such restriction, the subroutine could also accept a string (in which case, one easy way to validate would likely involve an eval() ).

      The code, $string =~ $re ;, will work in both cases:

      1. when $re is of type Regexp, as in $re=qr/\w/i,
      2. when $re is just a scalar for a string, as in
        • $re = '(?i-xsm:\w)', or
        • $re = '\w';
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