Backslash not working in regex when stored in an array in Perl

Question

I'm working on a simple lexical analyser for a project and I'm using an array to store regex patterns as strings.

I test each regex pattern individually to make sure I get the correct output when checking a line of a file.

The problem is, once I stored these strings in an array I started getting several errors. Mainly when searching for strings that start with backslash \\ .

This is my Perl code

#!/usr/bin/perl

@PATTERNS = ("\\begin", "\\end", "{", "}", "<\d+(\.\d+)?>", "<p>", "<\\p>", ".*", "%%+", "<%", "%>") ;

print "Enter some text: ";
chomp( $input = <> );

print test();

sub test() {

    my $arrSize = @PATTERNS;

    for ( my $i = 0; $i < $arrSize; $i++ ) {

        if ( $input =~ /$PATTERNS[$i]/gi ) {
            print "good input\n";
        }
        else {
            print "bad input\n";
        }
    }

}

This above is my test file to read lines I manually type in to check regex expression and give me a good input if it matches or bad input if it doesn't.

Perl continues to skip over my backslashes no matter how I use it in each string.

I'm using the default Perl v5.18.2 that is installed with Ubuntu 14.04.

Answer 1

Strings in double quotes are "interpolated" in Perl. Backslash has a special meaning here. If you want to store regular expressions in an array, it's better to use the qr// construct:

my @PATTERNS = ( qr/\\begin/,
                 qr/\\end/,
                 qr/{/,
                 qr/}/,
                 qr/<\d+(\.\d+)?>/,
                 qr/<p>/,
                 qr(</p>), # I assumed HTML/PHP, so replaced \p.
                 qr/.*/,
                 qr/%%+/,
                 qr/<%/,
                 qr/%>/,
               ) ;

You should use warnings , they would warn you against some mistakes you made:

Unrecognized escape \d passed through at /home/choroba/1.pl line 5.
Unrecognized escape \d passed through at /home/choroba/1.pl line 5.
main::test() called too early to check prototype at /home/choroba/1.pl line 10.