#!/usr/bin/perl -w
use strict;

my ($data, $res);

$data = '<a & href="somesite.html">test<\a>';

print "Before substitution: $data\n";

$res = $data =~ s/

        (                # Capture to $1 <a and 
            <a\s         # a space character
        )

        (?:              # Non-capturing parens
            [^>]*        # stuff between a and href
        )                

        (
            href\s*      # href followed by spaces 
        ) 
 
        (                
            =\s*         # Equals followed by spaces

            (            
                ["']+    # Open quote character
            )       

            (              
                [^"']+   # Non open quote character
            )              

            (?:
                \4       # Close quote character 
            )          

        )

        (                
            >            # Not final close angle bracket
        )

        (
            [^>]+        # Up to closing angle bracket
            >            # Final close angle bracket
        )
        /$1$2$3$6$7/x;

print "no match\n" if ($res eq "");

print "After substitution:  $data\n";