Javascript greedy regex appears non-greedy

Question

I'd like to match a three-part string. The first part consists of one or more a characters, the second part consists of one or more b characters, and the third part consists either of zero or more c characters or zero or more C characters, but not a mix of c and C.

As such, I wrote the following regular expression:

/a+b+(C*|c*)/

And immediately noticed that it fails to greedily match the trailing cs in the following string:

aaaaabbcc

Wrapping the inner clauses of the or clause does not fix the unexpected behavior:

/a+b+((C*)|(c*))/

But interestingly both regular expressions match the following, where the C characters match the first clause of the or:

aaaaabbCC

The following regular expression captures the semantics accurately, but I'd like to understand why the original regular expression behaves unexpectedly.

/a+b+(([Cc])\2*)?/

Do you mean it does not work because the c is not matched? However, C* did, that is why c* did not even get tested. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 7, 2016 at 20:41

skeggse · Accepted Answer · 2016-08-07 21:52:09Z

5

Your regex doesn't work because first it tries C*, which matches the empty string, so it has satisfied the or clause. Then it won't try to check if c* can match more characters.

Here's a regular expression which does match the string as intended:

/a+b+(C+|c+)?/

That is, if it finds a C it will match as many more C as possible, if it finds a c it will match as many more c as possible. But finding C or c is optional.

edited Aug 7, 2016 at 21:52

skeggse

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answered Aug 7, 2016 at 20:49

Oriol

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csabinho · Accepted Answer · 2016-08-07 20:42:11Z

2

You have to place the * outside of the bracket!

answered Aug 7, 2016 at 20:42

csabinho

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1 Comment

skeggse Over a year ago

No, that would match aaaaabbcCcCc, which I do not want.

Hitmands · Accepted Answer · 2016-08-07 20:42:30Z

1

var input = "aaaaabbc";

// if you want to pick up c
console.log(/a+b+(c|C)*/.exec(input).pop());

answered Aug 7, 2016 at 20:42

Hitmands

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4 Comments

skeggse Over a year ago

No, that would match aaaaabbcCcCc, which I do not want.

skeggse Over a year ago

Yep, I realized it was a little lacking in detail. The point is that the regex appears to be treating the two clauses of the or clause inconsistently.

skeggse Over a year ago

If you have a critique of my question, please make a specific comment. I have read that page in the past, and merely linking to it does not help me improve my question.

Hitmands Over a year ago

I don't understand what you are looking for, probably you need to pick up the c, probably all the string, probably the c could be optionally matched... I can't help you, the question is your, so, you need to clarify what you want.

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