CODEWITHSUNDEEP
regexgreedynon-greedyregex-greedyalternation
Stomp
Stomp

Reputation: 920

Regex is behaving lazy, should be greedy

I thought that by default my Regex would exhibit the greedy behavior that I want, but it is not in the following code:

 Regex keywords = new Regex(@"in|int|into|internal|interface");
 var targets = keywords.ToString().Split('|');
 foreach (string t in targets)
    {
    Match match = keywords.Match(t);
    Console.WriteLine("Matched {0,-9} with {1}", t, match.Value);
    }

Output:

Matched in        with in
Matched int       with in
Matched into      with in
Matched internal  with in
Matched interface with in

Now I realize that I could get it to work for this small example if I simply sorted the keywords by length descending, but

So my question is: Why is this being lazy and how do I fix it?

Upvotes: 12

Views: 1184

Answers (3)

Max Shawabkeh
Max Shawabkeh

Reputation: 38643

Laziness and greediness applies to quantifiers only (?, *, +, {min,max}). Alternations always match in order and try the first possible match.

Upvotes: 13

Jason D
Jason D

Reputation: 2303

It looks like you're trying to word break things. To do that you need the entire expression to be correct, your current one is not. Try this one instead..

new Regex(@"\b(in|int|into|internal|interface)\b");

The "\b" says to match word boundaries, and is a zero-width match. This is locale dependent behavior, but in general this means whitespace and punctuation. Being a zero width match it will not contain the character that caused the regex engine to detect the word boundary.

Upvotes: 6

Jeras
Jeras

Reputation: 138

According to RegularExpressions.info, regular expressions are eager. Therefore, when it goes through your piped expression, it stops on the first solid match.

My recommendation would be to store all of your keywords in an array or list, then generate the sorted, piped expression when you need it. You would only have to do this once too as long as your keyword list doesn't change. Just store the generated expression in a singleton of some sort and return that on regex executions.

Upvotes: 3

Related Questions