CODEWITHSUNDEEP
javascriptregexperformancehex
user4449804
user4449804

Reputation:

This simply regular expression is really slow, why?

I have written this really simple regular expression which checks if the whole string represents one or more hexadecimal numbers (optionally starting with "0x", separated by comma, minus sign or a whitespace character):

/^((0x)?[0-9A-F]+[,\-\s]*)+$/i

To me, this regex is very straightforward, but this simple test crashs Firefox and Edge at all, Chrome takes 100% CPU usage for about 15 seconds and then returns the expected result "false":

/^((0x)?[0-9A-F]+[,\-\s]*)+$/i.test("012345678901234567890123456789_0");

Has anyone an idea what's wrong here?

Upvotes: 2

Views: 1687

Answers (1)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 626950

It is a usual case of a catastrophic backtracking (see your regex demo) when a group contains two subpatterns with one of them being optional.

You need to regroup the pattern as

/^(?:0x)?[0-9A-F]+(?:[,\s-]+(?:0x)?[0-9A-F]+)*$/i

Now, it will match:

  • ^ - start of string
  • (?:0x)?[0-9A-F]+ - hex number
  • (?:[,\s-]+(?:0x)?[0-9A-F]+)* - zero or more sequences of
    • [,\s-]+ - 1+ space, , or - symbols (remove + if only 1 must be matched)
    • (?:0x)?[0-9A-F]+ - hex number
  • $ - end of string.

See the regex demo.

Upvotes: 3

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