Liberal Regex Pattern for All URLs

The regex patterns in this gist are intended to match any URLs,
including "mailto:foo@example.com", "x-whatever://foo", etc. For a
pattern that attempts only to match web URLs (http, https), see:
https://gist.github.com/gruber/8891611


# Single-line version of pattern:

(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))


# Multi-line commented version of same pattern:

(?xi)
\b
(							# Capture 1: entire matched URL
  (?:
    [a-z][\w-]+:				# URL protocol and colon
    (?:
      /{1,3}						# 1-3 slashes
      |								#   or
      [a-z0-9%]						# Single letter or digit or '%'
      								# (Trying not to match e.g. "URI::Escape")
    )
    |							#   or
    www\d{0,3}[.]				# "www.", "www1.", "www2." … "www999."
    |							#   or
    [a-z0-9.\-]+[.][a-z]{2,4}/	# looks like domain name followed by a slash
  )
  (?:							# One or more:
    [^\s()<>]+						# Run of non-space, non-()<>
    |								#   or
    \(([^\s()<>]+|(\([^\s()<>]+\)))*\)	# balanced parens, up to 2 levels
  )+
  (?:							# End with:
    \(([^\s()<>]+|(\([^\s()<>]+\)))*\)	# balanced parens, up to 2 levels
    |									#   or
    [^\s`!()\[\]{};:'".,<>?«»“”‘’]		# not a space or one of these punct char
  )
)