C# multiple string match

Question

I need C# string search algorithm which can match multiple occurance of pattern. For example, if pattern is 'AA' and string is 'BAAABBB' Regex produce match result Index = 1, but I need result Index = 1,2. Can I force Regex to give such result?

pattern '(?=A)' gives good results but enormously exten calc time. I have a string with 20M characters and calc speed is very important. Does anyone has other solution? Thanks. — , Jan 09 '09 at 12:50
"(?=A)" doesn't do what you want anyway; have you tried "A(?=A)" like AnthonyWJones suggested? — Alan Moore, Jan 10 '09 at 16:56

score 13 · Answer 1 · answered Jan 08 '09 at 10:26

13

Use a lookahead pattern:-

"A(?=A)"

This finds any A that is followed by another A without consuming the following A. Hence AAA will match this pattern twice.

answered Jan 08 '09 at 10:26

AnthonyWJones

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Sani Huttunen · Answer 2 · 2009-01-08T12:10:40.463

4

To summarize all previous comments:

Dim rx As Regex = New Regex("(?=AA)")
Dim mc As MatchCollection = rx.Matches("BAAABBB")

This will produce the result you are requesting.

EDIT:
Here is the C# version (working with VB.NET today so I accidentally continued with VB.NET).

Regex rx = new Regex("(?=AA)");
MatchCollection mc = rx.Matches("BAAABBB");

edited Jan 08 '09 at 12:10

answered Jan 08 '09 at 10:52

Sani Huttunen

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score 0 · Answer 3 · answered Jan 08 '09 at 10:24

0

Any regular expression can give an array of MatchCollection

answered Jan 08 '09 at 10:24

Dror

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score 0 · Answer 4 · answered Jan 08 '09 at 10:26

0

Regex.Matches() http://msdn.microsoft.com/en-us/library/system.text.regularexpressions.matchcollection.aspx

answered Jan 08 '09 at 10:26

Android

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Lonzo · Answer 5 · 2009-01-08T15:05:36.837

Try this:

       System.Text.RegularExpressions.MatchCollection  matchCol;
       System.Text.RegularExpressions.Regex regX = new System.Text.RegularExpressions.Regex("(?=AA)");

        string index="",str="BAAABBB"; 
        matchCol = regX.Matches(str);
        foreach (System.Text.RegularExpressions.Match mat in matchCol)
            {
                index = index + mat.Index + ",";
            }

The contents of index are what you are looking for with the last comma removed.

score 0 · Answer 6 · answered Jan 10 '09 at 17:19

Are you really looking for substrings that are only two characters long? If so, searching a 20-million character string is going to be slow no matter what regex you use (or any non-regex technique, for that matter). If the search string is longer, the regex engine can employ a search algorithm like Boyer-Moore or Knuth-Morris-Pratt to speed up the search--the longer the better, in fact.

By the way, the kind of search you're talking about is called overlapping matches; I'll add that to the tags.

C# multiple string match

6 Answers6