Matching a capturing group or failng the match if missing in Javascript regex

Question

I am trying to grab a digit (5) followed by a possible - or (space) proceeding a sequence of 3 digits, followed finally by a single digit. If the first group doesn't match at all, then only return the other sequences of digits.

^(5\-? ?)?(\d{3})(\d)$

This looks right to me and doesn't throw any errors, but it's giving the 5 back:

"5489" -> ()("548")("9")

Where I would actually not want this expression to return a match for this pattern.

So a quick search brought me to possessive expressions and a lot of articles about your ex. From what I'm reading, this looks like it should work:

^(5\-? ?)?+(\d{3})(\d)$

But Javascript does not like that as a regular expression.

Is there a way to do a greedy possessive capture group in Javascript, or simulate it in this situation?

If you have a look at the ECMAScript 5 docs, you will see there is no support for possessive quantifiers. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jun 26, 2017 at 19:51
If it's just not 5 then this ^(5\-? ?)?((?!5)\d{3})(\d)$ — user557597
– user557597, Commented Jun 26, 2017 at 19:52
Try ^(?!5\d{3}$)(5\-? ?)?(\d{3})(\d)$. It will fail a match at once if the string starts with 5 and then has 3 digits. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jun 26, 2017 at 19:55
@RandyHall - There is no reason to force anything. You've got a fixed amount of digits to match at the end, plus you're using anchors ^$. The conclusion is that if you have 4 digits only they will be matched at the end because its a tenent of the regex design. If you don't want to match 4 digits starting with a 5, then it's an extra assertion, but has nothing to do with forcing a capture of the first group. I'm just stating this as a comment because it's an X/Y problem, and I don't like to post for no reason. — user557597
– user557597, Commented Jun 26, 2017 at 20:05

revo · Accepted Answer · 2017-06-26 20:08:07Z

3

You can simulate possessive quantifier functionality by taking advantage of lookaround qualities:

^(?=(...))\1

Regex:

^(?=((5\-? ?)?))\1(\d{3})(\d)$

Live demo

answered Jun 26, 2017 at 20:08

revo

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user557597 Over a year ago

+1 I think this is the better answer. One, that it simulates possessive, two, that it won't match 5890 but will match 55890.

Wiktor Stribiżew · Accepted Answer · 2017-06-26 20:00:11Z

1

If you have a look at the ECMAScript 5 docs, you will see there is no support for possessive quantifiers.

You may use

^(?!5\d{3}$)(5-? ?)?(\d{3})(\d)$

See the regex demo. The (?!5\d{3}$) negative lookahead will fail a match at once if the string starts with 5 and then has 3 digits.

Details:

^ - start of string
(?!5\d{3}$) - there cannot be 5 and then 3 digits and the end of string immediately to the right of the current location
(5-? ?)? - an optional sequence of 5, then an optional - and then an optional space
(\d{3}) - 3 digits
(\d) - one digit
$ - end of string.

answered Jun 26, 2017 at 20:00

Wiktor Stribiżew

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