Javascript Regex: ignore subgroup when matching

Question

I'm trying to match a string with an optional part in the middle.

Example strings are:

20160131_abc.pdf
20160131_abc_xx.pdf
20160131_def.pdf

The result should include the name of the file (w/o the optional _xx).

What I have so far:

/[0-9]{8}_(abc(_xx)?|def)\.pdf/i

This kind of works but will return the name as abc_xx for the second string - I only want the abc part of it. Is it somehow possible to ignore the subgroup?

What is the problem if you simply replace "_xx" without a regex? And then use split. — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Apr 14, 2016 at 9:38
Exactly my thought. Can't my_string = my_string.replace("_xx", ""); do the job? — Kulvar
– Kulvar, Commented Apr 14, 2016 at 9:41
I found a way, please check: [0-9]{8}_(abc|def(?=\.pdf))(?:_xx)?\.pdf — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Apr 14, 2016 at 9:42

Wiktor Stribiżew · Accepted Answer · 2016-04-14 09:44:17Z

1

You can restrict the def part with a (?=\.pdf) lookahead that will require .pdf to appear right after def if there is def before the .pdf and add the optional group (?:_xx)? before the .pdf:

[0-9]{8}_(abc|def(?=\.pdf))(?:_xx)?\.pdf

See the regex demo

Explanation:

[0-9]{8} - 8 digits
_ - underscore
(abc|def(?=\.pdf)) - Capture group 1 matching abc or def (def is only matched if .pdf follows it immediately)
(?:_xx)? - optional _xx part that can only appear in the match (not in the capture) if preceded with abc
\.pdf - literal .pdf substring

answered Apr 14, 2016 at 9:44

Wiktor Stribiżew

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2 Comments

Casimir et Hippolyte Over a year ago

Note that only checking the dot in the lookahead suffices.

Wiktor Stribiżew Over a year ago

Yes, it is possible to omit pdf in the lookahead. [0-9]{8}_(abc|def(?=\.))(?:_xx)?\.pdf is enough, but the gain is tiny.

ptrk · Accepted Answer · 2016-04-14 09:55:14Z

0

You can use non-capturing groups in the regex and then "implode" the match results:

var re = /([0-9]{8}_)(abc|def)(?:_xx)?(\.pdf)/;
var tests = [
  '20160131_abc.pdf',
  '20160131_abc_xx.pdf',
  '20160131_def.pdf'
];
var container = document.getElementById('container');
tests.forEach(function(test){
  var match = test.match(re);
  var fileName = match.slice(1).join('');
  container.innerHTML += "test:" + test + " &rarr; ";
  container.innerHTML += fileName + "<br/>";
});

See fiddle

answered Apr 14, 2016 at 9:55

ptrk

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1 Comment

Philipp Kyeck Over a year ago

this also matches 20160131_def_xx.pdf and it shouldn't

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