Dynamically capture regular expression match in Perl

Question

I'm trying to dynamically catch regex matching in Perl. I've known that eval will help me do this but I may be doing something wrong.

Code:

use strict;
use warnings;

my %testHash = (
    '(\d+)\/(\d+)\/(\d+)'   =>  '$1$2$3'
);

my $str = '1/12/2016';

foreach my $pattern (keys (%testHash)) {
    my $value = $testHash{$pattern};
    my $result;

    eval {
        local $_ = $str;
        /$pattern/;
        print "\$1 - $1\n";
        print "\$2 - $2\n";
        print "\$3 - $3\n";
        eval { print "$value\n"; }
    }
}

Is it also possible to store captured regex patterns in an array?

Answer 1

I believe what you really want is a dynamic version of the following:

say $str =~ s/(\d+)\/(\d+)\/(\d+)/$1$2$3/gr;

String::Substitution provides what we need to achieve that.

use String::Substitution qw( gsub_copy );

for my $pattern (keys(%testHash)) {
   my $replacement = $testHash{$pattern};
   say gsub_copy($str, $pattern, $replacement);
}

Note that $replacement can also be a callback. This permits far more complicated substitutions. For example, if you wanted to convert 1/12/2016 into 2016-01-12, you could use the following:

'(\d+)/(\d+)/(\d+)' => sub { sprintf "%d-%02d-%02d", @_[3,1,2] },

---

To answer your actual question:

use String::Substitution qw( interpolate_match_vars last_match_vars );

for my $pattern (keys(%testHash)) {
   my $template = $testHash{$pattern};

   $str =~ $pattern   # Or /$pattern/ if you prefer
      or die("No match!\n");

   say interpolate_match_vars($template, last_match_vars());
}

Answer 2

I am not completely sure what you want to do here, but I don't think your program does what you think it does.

You are useing eval with a BLOCK of code. That's like a try block. If it dies inside of that eval block, it will catch that error. It will not run your string like it was code. You need a string eval for that.

Instead of explaining that, here's an alternative.

This program uses sprintf and numbers the parameters. The %1$s syntax in the pattern says _take the first argument (1$) and format it as a string (%s). You don't need to localize or assign to $_ to do a match. The =~ operator does that on other variables for you. I also use qr{} to create a quoted regular expression (essentially a variable containing a precompiled pattern) that I can use directly. Because of the {} as delimiter, I don't need to escape the slashes.

use strict;
use warnings;
use feature 'say'; # like print ..., "\n"

my %testHash = (
    qr{(\d+)/(\d+)/(\d+)}         => '%1$s.%2$s.%3$s',
    qr{(\d+)/(\d+)/(\d+) nomatch} => '%1$s.%2$s.%3$s',
    qr{(\d+)/(\d+)/(\d\d\d\d)}    => '%3$4d-%2$02d-%1$02d',
    qr{\d}                        => '%s', # no capture group
);

my $str = '1/12/2016';

foreach my $pattern ( keys %testHash ) {
    my @captures = ( $str =~ $pattern );

    say "pattern: $pattern";

    if ($#+ == 0) {
        say "  no capture groups";
        next;
    }

    unless (@captures) {
        say "  no match";
        next;
    }

    # debug-output
    for my $i ( 1 .. $#- ) {
        say sprintf "  \$%d - %s", $i, $captures[ $i - 1 ];
    }

    say sprintf $testHash{$pattern}, @captures;
}

I included four examples:

• The first pattern is the one you had. It uses %1$s and so on as explained above.
• The second one does not match. We check the number of elements in @captured by looking at it in scalar context.
• The third one shows that you can also reorder the result, or even use the sprintf formatting.
• The last one has no capture group. We check by looking at the index of the last element ($# as the sigil for arrays that usually have an @ sigil) in @+, which holds the offsets of the ends of the last successful submatches in the currently active dynamic scope. The first element is the end of the overall match, so if this only has one element, we don't have capture groups.

The output for me is this:

pattern: (?^:(\d+)/(\d+)/(\d\d\d\d))
  $1 - 1
  $2 - 12
  $3 - 2016
2016-12-01
pattern: (?^:(\d+)/(\d+)/(\d+) nomatch)
  no match
pattern: (?^:\d)
  no capture groups
pattern: (?^:(\d+)/(\d+)/(\d+))
  $1 - 1
  $2 - 12
  $3 - 2016
1.12.2016

Note that the order in the output is mixed up. That's because hashes are not ordered in Perl, and if you iterate over the keys in a hash without sort the order is random.

Answer 3

Apologies! I realized both my question and sample code were both vague. But after reading your suggestions I came of with the following code. I haven't optimized this code yet and there is a limit to the replacement.

foreach my $key (keys %testHash) {

    if ( $str =~ $key ) {
        my @matchArr = ($str =~ $key); # Capture all matches

        # Search and replace (limited from $1 to $9)
        for ( my $i = 0; $i < @matchArr; $i++ ) {
            my $num = $i+1;
            $testHash{$key} =~ s/\$$num/$matchArr[$i]/;
        }

        $result = $testHash{$key};

        last;
    }
}

print "$result\n";

Answer 4

Evaluing the regexp in list context returns the matches. so in your example:

use Data::Dumper; # so we can see the result
foreach my $pattern (keys (%testHash)) {
    my @a = ($str =~/$pattern/);
    print Dumper(\@a);
}

would do the job.

HTH Georg

Answer 5

Is it also possible to store captured regex patterns in an array?

Of course it is possible to store captured substrings in an array:

#!/usr/bin/env perl

use strict;
use warnings;

my @patterns = map qr{$_}, qw{
    (\d+)/(\d+)/(\d+)
};

my $str = '1/12/2016';

foreach my $pattern ( @patterns ) {
    my @captured = ($str =~ $pattern)
        or next;
    print "'$_'\n" for @captured;
}

Output:

'1'
'12'
'2016'

I do not quite understand what you are trying to do with combinations of local, eval EXPR and eval BLOCK in your code and the purpose of the following hash:

my %testHash = (
    '(\d+)\/(\d+)\/(\d+)'   =>  '$1$2$3'
);

If you are trying to codify that this pattern should result in three captures, you can do that like this:

my @tests = (
    {
        pattern => qr{(\d+)/(\d+)/(\d+)},
        ncaptures => 3,
    }
);

my $str = '1/12/2016';

foreach my $test ( @tests ) {
    my @captured = ($str =~ $test->{pattern})
        or next;
    unless (@captured == $test->{ncaptures}) {
        # handle failure
    }
}

See this answer to find out how you can automate counting the number of capture groups in a pattern. Using the technique in that answer:

#!/usr/bin/env perl

use strict;
use warnings;

use Test::More;

my @tests = map +{ pattern => qr{$_}, ncaptures => number_of_capturing_groups($_) }, qw(
    (\d+)/(\d+)/(\d+)
);

my $str = '1/12/2016';

foreach my $test ( @tests ) {
    my @captured = ($str =~ $test->{pattern});
    ok @captured == $test->{ncaptures};
}

done_testing;

sub number_of_capturing_groups {
    "" =~ /|$_[0]/;
    return $#+;
}

Output:

ok 1
1..1