How to replace only capturing group in Regex.Replace?

Question

I have dictionary with key as pattern and replacement as value. Each pattern have one capturing group. I would like to replace ONLY capturing group with the replacement. My attempt is the following, but of course it's replacing whole pattern. I am limited to .NET 3.5. Not sure if I am on right track.

        string xml = "abc def ghi blabla horse 123 jakljd alj ldkfj s;aljf kljf sdlkj flskdjflskdjlf lskjddhcn guffy";
        Dictionary<string, string> substitutions = new Dictionary<string, string> 
        { 
            {"abc (.+) ghi", "AAA"},
            {"kljf (.+) flskdjflskdjlf", "BBB"}
        };

        foreach(KeyValuePair<string, string> entry in substitutions)
        {
            xml = Regex.Replace(xml, entry.Key, delegate(Match m) { return m.Groups[1].Value; });
            Console.WriteLine(xml);
        }

The final string should like this:

"abc AAA ghi blabla horse 123 jakljd alj ldkfj s;aljf BBB sdlkj flskdjflskdjlf lskjddhcn guffy"

vks · Accepted Answer · 2015-09-03 09:38:20Z

1

You need to use lookarounds.

"(?<=abc ).+(?= ghi)", "AAA"

This will enable you to replace the words you want.You dont need capture group

answered Sep 3, 2015 at 9:38

vks

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Comments

Saeb Amini · Accepted Answer · 2015-09-03 09:43:28Z

0

Use positive loohbehind and lookaheads:

string xml = "abc def ghi blabla horse 123 jakljd alj ldkfj s;aljf kljf sdlkj flskdjflskdjlf lskjddhcn guffy";
Dictionary<string, string> substitutions = new Dictionary<string, string> 
{ 
    {@"(?<=abc\s).+(?=\sghi)", "AAA"},
    {@"(?<=kljf\s).+(?=\sflskdjflskdjlf)", "BBB"}
};

foreach (KeyValuePair<string, string> entry in substitutions)
{
    xml = Regex.Replace(xml, entry.Key, entry.Value);
    Console.WriteLine(xml);
}

They are zero-width assertions, i.e., they are required to satisfy the match, but won't be included in the result.

answered Sep 3, 2015 at 9:43

Saeb Amini

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3 Comments

Wiktor Stribiżew Over a year ago

This basically the same what vks suggests.

vks Over a year ago

Isnt this the same as my answer?

Saeb Amini Over a year ago

@stribizhev Yes, the idea is the same, however I started writing this answer about 30 seconds after the question was asked, and I usually take my time writing a relatively complete answer, disliking the fastest gun in the west approach, and during that time fast guns might shoot an answer with the same concept, however I still post my answer when I'm done FWIW.

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