Javascript regex backreference indexing

Question

I need a regex to match a string that

either starts with a special character like [#.>+~] and is followed by a lowercase ASCII word, or that
consists only of a different special character like *.

The special character should be captured in group number 1, the following word (or, in the second case, the empty string) in group number 2.

I can handle the first case with /^([#\.>+~]?)([a-z]+)$/, but how can I get the second case into this regex to achieve the following results:

"#word"  -> 1 => "#", 2 => "word"
"~word"  -> 1 => "~", 2 => "word"
"##word" -> no match
"+#word" -> no match
"!word"  -> no match
"*word"  -> no match
"word"   -> 1 => "",  2 => "word"
"*"      -> 1 => "*", 2 => ""
"**"     -> no match
"*word"  -> no match

So you want to match match1match2 if it occurs at the start of the string or at the end of the string? I suspect you've given us a simplified regex than the one you're actually using. Could you post that instead? Perhaps there's a better way to achieve what you want. — Tim Pietzcker
– Tim Pietzcker, Commented May 27, 2013 at 13:31
To make sure /^([#\.>+~]?)([a-z]+)$/ matches both "+word", "word" and things like single special characters (other than those specified in the first match1 group), which I would put in the second match1, such as "*". Results: ("+word" -> 1 => "+", 2 => "word"), ("word" -> 1 => "", 2 => "word"), ("*" -> 1 => "*", 2 => null) — user2180613
– user2180613, Commented May 27, 2013 at 13:31
Sorry, I don't get it. The comment field is unsuitable for posting code. Please edit your question instead. — Tim Pietzcker
– Tim Pietzcker, Commented May 27, 2013 at 13:32

Tim Pietzcker · Accepted Answer · 2013-05-27 14:01:55Z

1

This regex should do what you need:

/^([#~.>+](?=[a-z]+$)|[*](?=$))([a-z]*)$/

See it on regex101.com

Explanation:

^          # Start of string
(          # Match and capture in group number 1:
 [#~.>+]   # Either: one "special character"
 (?=       #  but only if it's followed by
  [a-z]+   #   at least one lowercase ASCII letter
  $        #   and the end of the string.
 )         #  End of lookahead
|          # OR
 [*]       #  one (different) special character
 (?=$)     #  but only if the string ends right after it.
)          # End of the first capturing group
(          # Match and capture in group number 2:
 [a-z]*    # Zero or more ASCII lowercase letters
)          # End of the second capturing group
$          # End of string

edited May 27, 2013 at 14:01

answered May 27, 2013 at 13:28

Tim Pietzcker

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