a simple string substitution does not work

Question

Below is my codes:

my $string1 = '<td><a href="http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3">abcdefg</a><br />(123456)</td>';
my $string2 = 'http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3';


print "Before string substitution:\n$string1\n";
$string1 =~ s/$string2//;
print "After string substitution:\n$string1\n"; 

And the actual output:

Before string substitution:
<td><a href="http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3">abcdefg</a><br />(123456)</td>
After string substitution:
<td><a href="http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3">abcdefg</a><br />(123456)</td> 

What I expect:

Before string substitution:
<td><a href="http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3">abcdefg</a><br />(123456)</td>
After string substitution:
<td><a href="">abcdefg</a><br />(123456)</td> 

could someone please tell me what is wrong in my code?

Thanks.

Answer 1

That problem can be fixed by adding two characters to your script. What you need is to escape meta characters in $string2 :

$string1 =~ s/\Q$string2//;

The character that causes the match to fail is the question mark ? , which unescaped here ...aspx?... means "match 0 or 1 of the character 'x'". The characters . are wildcards that match anything except newline, which may cause false positive matches. The slashes / , while being meta characters due to being the delimiter of the substitution operator s/// , do not need to be escaped since they are embedded in a string.

Escaping meta characters is most easily done with the \\Q ... \\E escape sequence, inside a regex, or with quotemeta .

It is not a good idea to try and escape these kinds of strings manually, especially if literal matches are all that is required.

Answer 2

Since you are putting in characters that are considered special characters by perl regex, you must escape them out like this:

my $string2 = 'http:\/\/www\.aaa\.com\/downloads\/details\.aspx\?FamilyID=a1b2c3';

Then the expected output will show up when you run your program:

<td><a href="http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3">abcdefg</a><br />(123456)</td>
After string substitution:
<td><a href="">abcdefg</a><br />(123456)</td>

To escape these characters from your string, it is best to just use perl's quotemeta function:

my $string2 = quotemeta('http://www.aaa.com/downloads/details.aspx?FamilyID=a1b2c3');

This will escape the special characters for you and then your regex replace will work fine.

Since you're having issues because of non-escaped regex characters, this solution might be simpler since it does not require you to escape any characters:

substr($string1, index($string1,$string2), length($string2)) = '';

This is based off of this example:

my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'

found in the perldocs for substr .