in reply to string substitution within regular expressions?

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/";
}

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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.

Replies are listed 'Best First'.
Re^2: string substitution within regular expressions?
by Anonymous Monk on Feb 09, 2005 at 03:29 UTC
    Okay, utilizing the code suggested above, I get the expected behaviour. If I change the if-statement to the following, I get the errors noted further below. 
    #!/usr/bin/perl

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

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

findString($s);

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

#   print "found\n" if $line =~ eval "qr/$regex/";
    if ($line =~ eval "qr/$regex/") {
        print "found\n";
    }
}

    [download]
    Do the following errors make sense? Yes, I'm using Perl 5.005, but I cannot change this at this point. 
    Scalar found where operator expected at (eval 1) line 1, at end of lin
+e
        (Missing operator before ?)
Backslash found where operator expected at (eval 1) line 1, near "$mon
+\"
        (Missing operator before \?)
found

    [download]
    Placing curly braces around the qr() string pacifies the interpreter, but I no longer see the expected answer. Thanks for any insight.
[reply]
[d/l]
[select]
      The errors don't make any sense to me, given that it works in the non-block if and only fails with block-form if. Does this work any better? Does it fail before the if? 
      sub findString($) {
    my $line = shift;
    my $compiled_regex = eval "qr/$regex/";

    if ($line =~ /$compiled_regex/) {
        print "found\n";
    }
}

      [download]
      Caution: Contents may have been coded under pressure.
[reply]
[d/l]
        Works like a champ. Thanks!
[reply]
Re^2: string substitution within regular expressions?
by Anonymous Monk on Feb 08, 2005 at 23:40 UTC
    Thanks for your suggestions. In regards to your first answer using eval, isn't the e switch to regular expressions provide the same functionality?
[reply]
      There is no e switch for regular expressions. The e switch is for substitutions, and applies to the replacement, not to the pattern.
      Caution: Contents may have been coded under pressure.
[reply]

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