Im trying to validate a username with RegEx in Javascript and while the result of .test is true all works fine, but (somtimes) when i use a string that doesnt match, my app breaks and i get the error Too much recursion. I want to know where in my RegEx (Or code) have i gone wrong to cause such a thing.

This is my code (RegEx):

console.log(/^[A-Za-zÀ-ÖØ-öø-ÿ]([ _]?[A-Za-z0-9À-ÖØ-öø-ÿ] ) $/g.test("RegExr was created by gskinner_com_and is proudly  hosted by Media Temple"));

I want the correct string to start with a letter, then have any combination of letters and digits with spaces and underscores allowed between them but only one at a time.

The regex works in websites related to testing it, and on my own code, and i get the error when the invalid input is far from the beginning of the string, in fact my computer takes longer to calculate the result the further i put the error (Lets say a dot or 2 spaces after each other) till on a certain point i get the error. or the use of too many characters like What is the problem? position of the $ or the use of too many characters like À-ÖØ-öø-ÿ? and what can i do to fix it ?

CodePudding user response：

The catastrophic backtracking is due to the nested quantifiers in the group. If the pattern can not match, the nested quantifiers allow a lot of positions to backtrack to, still trying to get a match.

You could start the pattern matching a single char, and then optionally match one of the character class [A-Za-z0-9À-ÖØ-öø-ÿ]*

Then in the repeating group, match either a space or underscore [ _] followed by 1 or more times the already existing character class [A-Za-z0-9À-ÖØ-öø-ÿ]

This does not disable the backtracking, it takes out a nested quantifier to allow for less positions to backtrack to.

Note that repeating a capture group, captures the value of the last iteration. If you don't need the capture group value afterwards, you can make it a non capture group using (?: instead.

^[A-Za-zÀ-ÖØ-öø-ÿ][A-Za-z0-9À-ÖØ-öø-ÿ]*(?:[ _][A-Za-z0-9À-ÖØ-öø-ÿ] ) $

Regex demo

To also match a single word you can make the quantifier * for the group to match zero or more times:

^[A-Za-zÀ-ÖØ-öø-ÿ][A-Za-z0-9À-ÖØ-öø-ÿ]*([ _][A-Za-z0-9À-ÖØ-öø-ÿ] )*$

Regex demo