CSS Parsing error

[TheTrigger]

Referring to the example on codepen, the following code works but bugs if add a space or tab after the comma.

#letters:nth-letter(even), #letters:nth-letter(odd){

Its not possible too to make new line and insert spaces, single or multiple newline works as codepen example.

Trying with regex101.com it was working with (s)ingle line flag. (\s*(.*?)\s*\{(.*?)\} /sg)