How would I match `.scss`, `.sass`, `.css` but not `.ass`? (RegExp)

Also this will match .scss, .sass and .css only, it is very readable and self-explanatory

/\.(sc|sa|c)ss$/

Another way using alternation:

\.((?:s[ac]|c)ss)$

RegEx Demo

Here this non-capturing group (?:s[ac]|c) will match sa or sc or just c.

How about just

/\.(s?css|sass)$/

Regex doesn't need to be very complex to work. This is a lot easier to read for other programmers (i.e. you in about 3 months) and does the same.

Sure you can smush it more together, but why would you? Regex are complicated enough, keep 'm simple if you can :)

Demo

You can use

\.(?!a)(s?[ac]ss)$

See the regex demo. Details:

• \. - a dot
• (?!a) - the next char cannot be a
• (s?[ac]ss) - Group 1: an optional s, a or c and then ss
• $ - end of string.

Another regex that can work is

\.(s(?:css|ass)|css)$

See this regex demo. Details:

• \. - a dot
• (s(?:css|ass)|css) - s and then css or ass or css
• $ - end of string.

NOTE: if you have a dynamic, user-defined list of such fixed strings to match after a . at the end of string, you can build these regexes automatically using the code at the bottom of my answer.

You could just list the ones you want to match:

let rx = /\.css|\.sass|\.scss/; // alphabetized for future maintenance 

This isn't fancy, but it is very clear and easy to add more later.

I tested it here :

In your pattern \.([s]?[ac]ss)$ you match .ass because the leading s optional and the character class [ac] can match both characters.

Instead you could use lookarounds assertions, or use an alternation | to allow only certain alternatives.

Some other variations could be:

\.(s?c|sa)ss$

• \.Match a .
• ( Capture group 1
  • s?c|sa Match an optional s then match c or match sa
• ) Close group 1
• ss$ Match ss at the end of the string

Regex demo

\.(s[ac]|c)ss$

A variation on the previous pattern, now matching sa or sc or c

Regex demo

If in your environment the lookbehind assertion is supported:

\.s?[ac]ss$(?<!\.ass)

• \.s? Match a . and optional s
• [ac] Match either a or c
• ss$ Match ss at the end of the string
• (?<!\.ass) Negative lookbehind, assert not .ass to the left

Regex demo

Note that if you want a match only, you can also use a non capture group (?:...) instead.