Example 6.13: urlify

#!/usr/bin/perl
# urlify - wrap HTML links around URL-like constructs

$urls = '(http|telnet|gopher|file|wais|ftp)';
$ltrs = '\w';
$gunk = '/#~:.?+=&%@!\-';
$punc = '.:?\-';
$any  = "${ltrs}${gunk}${punc}";

while (<>) {
    s{
      \b                    # start at word boundary
      (                     # begin $1  {
       $urls     :          # need resource and a colon
       [$any] +?            # followed by on or more
                            #  of any valid character, but
                            #  be conservative and take only
                            #  what you need to....
      )                     # end   $1  }
      (?=                   # look-ahead non-consumptive assertion
       [$punc]*             # either 0 or more punctuation
       [^$any]              #   followed by a non-url char
       |                    # or else
       $                    #   then end of the string
      )
     }{<A HREF="$1">$1</A>}igox;
    print;
}