CODEWITHSUNDEEP
regex
muffel
muffel

Reputation: 7390

Regular Expressions: Unbalanced parenthesis error

Why does the following regular expression leads to an 'unbalanced parenthesis error'?

[x(?:[0-9])]

I know this one doesn't make sense. I am working on a larger expression and found this as a minimal working example that produces the error. I want to realize a choice with a nested inner choice.

Upvotes: 0

Views: 155

Answers (2)

Dane
Dane

Reputation: 1211

Your problem is that you are trying to do the whole thing inside a character class created by the square brackets [...]. What the Regex sees is

[x(?:[0-9]

As one character class you're trying to match, followed by

)]

Which makes no sense, since the right-parenthesis is meaningless without a left.

What you might be trying to do is match a character class followed by an uncaptured digit:

[x](?:[0-9])

If you're trying to force this inside the square brackets, then I'm guessing that you want to match several different scenarios, such as:

[xyz](?:[0-4])|[abc](?:[5-9])

Regular expression visualization

Debuggex Demo

Upvotes: 1

p.s.w.g
p.s.w.g

Reputation: 149050

[…] creates a character class, which matches a single character in the set of characters that you specify within the braces. Character classes cannot contain groups, quantifiers, assertions, etc. Everything inside the character class is interpreted as a literal character, character range, or a predefined character set (like \s but not .)

So your pattern recognizes this as a character class [x(?:[0-9] (which matches a single x, (, ?, :, [, or any character from 0 to 9), followed by )]. But the ) is not escaped and is not matched with a corresponding (, and so it produces an error.

It's likely you just want:

x[0-9]

This will match an x followed by a single digit from 0 to 9.

Upvotes: 3

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