What does (? I do in regex?

All modes after the minus sign will be turned off. E.g. (? i-sm) turns on case insensitivity, and turns off both single-line mode and multi-line mode . Not all regex flavors support this.

If you want only part of the regex to be case insensitive (as my original answer presumed), then you have two options:

By default, the comparison of an input string with any literal characters in a regular expression pattern is case sensitive , white space in a regular expression pattern is interpreted as literal white-space characters, and capturing groups in a regular expression are named implicitly as well as explicitly.15 Eyl 2021