Regex exactly n OR m times

There is no single quantifier that means "exactly m or n times". The way you are doing it is fine.

An alternative is:

X{m}(X{k})?

where m < n and k is the value of n-m.

TLDR; (?<=[^x]|^)(x{n}|x{m})(?:[^x]|$)

Looks like you want "x n times" or "x m times", I think a literal translation to regex would be (x{n}|x{m}). Like this https://regex101.com/r/vH7yL5/1

or, in a case where you can have a sequence of more than m "x"s (assuming m > n), you can add 'following no "x"' and 'followed by no "x", translating to [^x](x{n}|x{m})[^x] but that would assume that there are always a character behind and after you "x"s. As you can see here: https://regex101.com/r/bB2vH2/1

you can change it to (?:[^x]|^)(x{n}|x{m})(?:[^x]|$), translating to "following no 'x' or following line start" and "followed by no 'x' or followed by line end". But still, it won't match two sequences with only one character between them (because the first match would require a character after, and the second a character before) as you can see here: https://regex101.com/r/oC5oJ4/1

Finally, to match the one character distant match, you can add a positive look ahead (?=) on the "no 'x' after" or a positive look behind (?<=) on the "no 'x' before", like this: https://regex101.com/r/mC4uX3/1

(?<=[^x]|^)(x{n}|x{m})(?:[^x]|$)

This way you will match only the exact number of 'x's you want.

No, there is no such quantifier. But I'd restructure it to /X{m}(X{m-n})?/ to prevent problems in backtracking.

Here is the complete list of quantifiers (ref. http://www.regular-expressions.info/reference.html):

• ?, ?? - 0 or 1 occurences (?? is lazy, ? is greedy)
• *, *? - any number of occurences
• +, +? - at least one occurence
• {n} - exactly n occurences
• {n,m} - n to m occurences, inclusive
• {n,m}? - n to m occurences, lazy
• {n,}, {n,}? - at least n occurence

To get "exactly N or M", you need to write the quantified regex twice, unless m,n are special:

• X{n,m} if m = n+1
• (?:X{n}){1,2} if m = 2n
• ...