Regex to match all the strings between two identical strings

Question

E.g. I have this string -- This -- is -- one -- another -- comment -- I want the matched elements to be "This", "is", "one", "another", and "comment"

I was trying this regex --\s+([^--]+)\s+-- which gives me the matched elements as "This", "one" and "comment"

I have searched for other problems, they all provide solution like this i.e. #A# and I will get A but for #A#B# also I get A, but in this case I want both the elements A and B as both of them are between two # chars.

I am testing it for javascript regex, but I think solution should be irrespective of platform/language.

Try this --\s+([^--]+)\s+, and then remove last two -- manually: regexr.com/3fffo — Piyush Kumar Baliyan
– Piyush Kumar Baliyan, Commented Mar 7, 2017 at 9:56
This [^--]+ will prevent matching mind-breaking in -- mind-breaking -- — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Mar 7, 2017 at 9:57

Wiktor Stribiżew · Accepted Answer · 2019-09-23 08:39:17Z

In general, you need to use a pattern like

STRING([\s\S]*?)(?=STRING|$)

It will match STRING, then capture into Group 1 any zero or more chars, as few as possible, up to the first occurrence of STRING *stopping right before this word** because the (?=...) is a positive lookahead that, being a zero-width assertion, does not consume matched text or end of string.

A generic variation of the pattern is

STRING((?:(?!STRING)[\s\S])*)

It uses a tempered greedy token, (?:(?!STRING)[\s\S])*, that matches any char, 0 or more occurrences, that does not start a STRING char sequence.

To get all the substrings in the current solution, use a lookahead like

/--\s+([\s\S]*?)(?=\s+--)/g
                ^^^^^^^^^

See the regex demo.

Note that [^--]+ matches 1 or more symbols other than a -, it does not match any text that is not equal to --. [...] is a character class that matches a single character. To match any text of any length from one char up to the first occurrence of a pattern, you can rely on a [\s\S]*? construct: any 0+ chars, as few as possible (due to the lazy *? quantifier).

JS demo:

var s = '-- This -- is -- one -- another -- comment --';
var rx = /--\s+([\s\S]*?)(?=\s+--)/g;
var m, res=[];
while (m = rx.exec(s)) {
  res.push(m[1]);
}
console.log(res);

Accepted the answer ! Thanks for solution with positive look ahead.
Lookarounds are the natural way to match substrings that overlap. Also, the [\s\S] that matches any char can be replaced with the native JS regex [^] construct (not nothing), but it is not portable. [\s\S] will work almost everywhere.
i wonder why you used /--\s+([\s\S]*?)(?=\s+--)/g instead of /--\s+([\s\S]*?)\s+(?=--)/g is there any performace reason or it's just aesthetics?
@MaciejKozieja: I do not think this is critical here, but that is interesting. Surely, the \s+ can be moved outside the lookahead. If there is a real performance difference (the number of steps at regex101 does not actually prove any regex is better than another), a test in JS environment should be set up. See jsfiddle.net/pbL0cmsj/1 - almost no difference in performance.

Locco0_0 · Accepted Answer · 2021-03-08 14:11:16Z

0

To read all I would use positive look ahead:

const data = '-- This -- is -- one -- another -- comment --'

const readAll = data => {
  const regex =/--\s*(.*?)\s*(?=--)/g
  const found = []
  let temp
  while (temp = regex.exec(data)) {
    found.push(temp[1])
  }
  return found
}

console.log(readAll(data))

And to remove comments just do this:

const data = `-- This -- is -- one -- another -- comment -- this is not a comment`.replace(/--.*--/g, '')

console.log(data)

edited Mar 8, 2021 at 14:11

Locco0_0

3,5005 gold badges33 silver badges43 bronze badges

answered Mar 7, 2017 at 9:57

Maciej Kozieja

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5 Comments

Wiktor Stribiżew Over a year ago

Note that OP's [^--] matches line break chars, while your .*? will not.

Maciej Kozieja Over a year ago

If you want line break you can do (?:\n|.)*? because [^--] dont allow for usage of one - because it is not - character second dash means until

Wiktor Stribiżew Over a year ago

Maciek, never use (?:\n|.)*?. It will crash your browser one day. See my answer for a proper way to match any char with a JS regex.

Maciej Kozieja Over a year ago

So you use [\s\S] to get all characters including new lines etc thats clever :D

Wiktor Stribiżew Over a year ago

See more about that here.

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