Forums Scripts & Popups Why this regexp pattern throw an error?

That is because you're indeed having an invalid format, you need to have the same number of '(' and ')' for a given parameter of an identifier, you are using \) which is why there one more ')'.
Possible workaround:
- put the pattern in a local variable and pass the variable as the parameter (clearer in general)
- use the octal or hexa representation of the ')' character
- use $chr() and $+ directly