Variables are interpolated when the pattern is examined -- that is, if you used qr/^$wday..../, it would try to substitute at that point. You're using single quotes, instead. So far so good.

But when you actually use it, you have /$regex/, so $regex gets interpolated into the pattern you said it was. The dollar signs are end-of-string indicators. What you really want is to eval it. 

sub findString($) {
    my $line = shift;
    print "found\n" if $line =~ eval "qr/$regex/";
}

[download]
The usual caveats about knowing that the contents of a string-eval are safe apply.

You can also use the experimental delayed-evaluation feature, though it still requires the variables to be in scope: 

my $s = 'Tue Feb  8 11:11:11 2005: blah blah blah';

my ($wday, $mon, $mday, $year);

my $regex = qr/^(??{$wday}) (??{$mon})\s+(??{$mday}) \d{2}:\d{2}:\d{2}
+ (??{$year}):/;
$wday = 'Tue';
$mon = 'Feb';
$mday = '8';
$year = '2005';

sub findString;
findString($s);

sub findString($) {
    my $line = shift;

    print "found\n" if $line =~ /$regex/;
}

[download]
Caution: Contents may have been coded under pressure.

In reply to Re: string substitution within regular expressions? by Roy Johnson
in thread string substitution within regular expressions? by Anonymous Monk

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