Regex Error: Nothing to Repeat

Question

I\'m new to regular expressions in JavaScript, and I cannot get a regex to work. The error is:

Uncaught SyntaxError: Invalid regular expression: /(.

Answer 1

Javascript regular expressions don't support possessive quantifiers. You should try with the reluctant (non-greedy) ones: *? or +?

Answer 2

Try this, it will seperate the left hand side and the right hand side:

(.*x.*)=(.+)

Live demo

Answer 3

You can emulate a possessive quantifier with javascript (since you can emulate an atomic group that is the same thing):

a++ => (?>a+) => (?=(a+))\1

The trick use the fact that the content of a lookahead assertion (?=...) becomes atomic once the closing parenthesis reached by the regex engine. If you put a capturing group inside (with what you want to be atomic or possessive), you only need to add a backreference \1 to the capture group after.

About your pattern:

.*+x is an always false assertion (like .*+=): since .* is greedy, it will match all possible characters, if you make it possessive .*+, the regex engine can not backtrack to match the "x" after.

What you can do:

Instead of using the vague .*, I suggest to describe more explicitly what can contain each capture group. I don't think you need possessive quantifiers for this task.

Trying to split the string on operator can be a good idea too, and avoids to build too complex patterns.