!/usr/bin/perl

use strict;
use warnings;
my $text;

while($text=<DATA>){
        $text =~ s~<font color=("|')?((\w|\s)+)("|')?>(.*?)<\/font>~\[color=$2\]$5\[/color\]~ig;
        print $text . "\n";
}

# expected output:
# [color=blue]<i><b> <br>Some text[/color],  <br>an a

__DATA__
<font color="blue"><i><b> <br>Some text</font>,  <br>an a