Sometimes it's useful, sometimes it isn't. In my regex class, I have an example that is a complete tokenizer for a calculator program, in one regex. The calculator accepts integer and floating-point numerals, +, -, *, /, ^, and ** operators, = for equality, := for assignment, parentheses for operator grouping, and variable names. The tokenizer is simple and easy to read:

sub tokens {
  my @tokens = 
  split m{  (
              \*\* | :=      # ** or := operator 
            |
              [-+*/^()=]  # some other operator
            |
              [A-Za-z]\w+ # Identifier
            |
              \d*\.\d+(?:[Ee]\d+)?  # Decimal number
            |
              \d+         # Integer
            )
        },
       shift();
   return grep /\S/, @tokens;
}
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(To see what this does, pass it a string like (Foo := 12) + 37^2-42*bar.)

Now what would this look like without /x? It would be a lot harder to understand:

 sub tokens { 
  my @tokens = 
  split m{(\*\*|:=|[-+*/^()=]|[A-Za-z]\w+|\d*\.\d+(?:[Ee]\d+)?|\d+)|\s
++},
    shift(); 
  return grep /\S/, @tokens;
}
[download]