Regex - Match multiple times in a string

Question

I am trying to do a regex search on 'NNTSY` so that I can get two matches.

NNTS
NTSY

When I attempted to match using the pattern ?<NGrlyosylation>N[^P][ST][^P])", I am only getting one match, which is NNTS.

How can I use Regex to match NNTSY so that two matches can be found?

NOTE: Background info: Rosalind problem can be found here.

Here is my code.

        input = "NNTSY";
        Regex regex = new Regex("(?<NGrlyosylation>N[^P][ST][^P])", RegexOptions.Compiled | RegexOptions.IgnoreCase);
        MatchCollection matches = regex.Matches(input);
        foreach (Match match in matches)
        {
            // Need to add 1 to because match index is 0 based
            const int offset = 1;
            yield return match.Index + offset;
        }

For those who want the full source, check out the source here => github.com/dance2die/Demo.Rosalind/blob/master/Demo.Rosalind/… — dance2die
– dance2die, Commented May 22, 2016 at 4:37

rock321987 · Accepted Answer · 2016-05-22 04:52:01Z

2

Finding overlapping matches is usually not allowed in most programming languages (except few). So, I don't think there exists a pure regex way to solve this, but you can use Substring in C# with lookahead as

(?=N[^P][ST][^P]).

C# Code

string input = "NNTSY";
Regex regex = new Regex("(?=N[^P][ST][^P]).", RegexOptions.Compiled | RegexOptions.IgnoreCase);

Match match = regex.Match(input);

while (match.Success)
{
    Console.WriteLine(input.Substring(match.Index, 4));
    match = match.NextMatch();
}

Ideone Demo

edited May 22, 2016 at 4:52

answered May 22, 2016 at 4:24

rock321987

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3 Comments

dance2die Over a year ago

Thanks @rock321987. I wasn't aware of Regex lookahead concept so I was lost. At least the tests I created passes all cases and I am ready to move on.

rock321987 Over a year ago

@Sung glad, it helped..lookaheads are zero width-assertion..it means that they do not consume any character..the regex will be checked against the string without the matched string being consumed

rock321987 Over a year ago

@Sung . was not required in the regex..simple lookahead will suffice here..adding . will speed up the process a bit

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