Using Heregexes

Problem

You need to write a complex regular expression.

Solution

Use Coffeescript’s “heregexes” – extended regular expressions that ignore internal whitespace and can contain comments.

pattern = ///
  ^\(?(\d{3})\)? # Capture area code, ignore optional parens
  [-\s]?(\d{3})  # Capture prefix, ignore optional dash or space
  -?(\d{4})      # Capture line-number, ignore optional dash
///
[area_code, prefix, line] = "(555)123-4567".match(pattern)[1..3]
# => ['555', '123', '4567']

Discussion

Breaking up your complex regular expressions and commenting key sections makes them a lot more decipherable and maintainable. For example, changing this regex to allow an optional space between the prefix and line number would now be fairly obvious.

Whitespace characters in heregexes

Whitespace is ignored in heregexes – so what do you do if you need to match a literal ASCII space?

One solution is to use the @\s@ character class, which will match spaces, tabs and line breaks. If you only want to match a space, though, you’ll need to use \x20 to denote a literal ASCII space.