Updated @gruber's regex with a modified version that looks for 2-13 letters rather than trying to look for specific TLDs. Given the recent addition of ~1400 gTLDs, it may be time to give up on that front.

Liberal Regex Pattern for URLs

# Single-line version:
(?i)\b((?:https?:(?:/{1,3}|[a-z0-9%])|[a-z0-9.\-]+[.](?:[a-z]{2,13})(?:[^\s()<>{}\[\]]+|\([^\s()]*?\([^\s()]+\)[^\s()]*?\)|\([^\s]+?\))+(?:\([^\s()]*?\([^\s()]+\)[^\s()]*?\)|\([^\s]+?\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’])|(?:(?<!@)[a-z0-9]+(?:[.\-][a-z0-9]+)*[.](?:[a-z]{2,13})\b/?(?!@)))


# Commented multi-line version:

(?xi)
\b
(							# Capture 1: entire matched URL
  (?:
    https?:				# 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
    							# looks like domain name followed by a slash:
    [a-z0-9.\-]+[.]
    (?:[a-z]{2,13})
    /
  )
  (?:							# One or more:
    [^\s()<>{}\[\]]+						# Run of non-space, non-()<>{}[]
    |								#   or
    \([^\s()]*?\([^\s()]+\)[^\s()]*?\)  # balanced parens, one level deep: (…(…)…)
    |
    \([^\s]+?\)							# balanced parens, non-recursive: (…)
  )+
  (?:							# End with:
    \([^\s()]*?\([^\s()]+\)[^\s()]*?\)  # balanced parens, one level deep: (…(…)…)
    |
    \([^\s]+?\)							# balanced parens, non-recursive: (…)
    |									#   or
    [^\s`!()\[\]{};:'".,<>?«»“”‘’]		# not a space or one of these punct chars
  )
  |					# OR, the following to match naked domains:
  (?:
  	(?<!@)			# not preceded by a @, avoid matching foo@_gmail.com_
    [a-z0-9]+
    (?:[.\-][a-z0-9]+)*
    [.]
    (?:[a-z]{2,13})
    \b
    /?
    (?!@)			# not succeeded by a @, avoid matching "foo.na" in "[email protected]"
  )
)