How to Regex search/replace only first occurrence in a string in .NET?

Question

It seems the .NET Regex.Replace method automatically replaces all matching occurrences. I could provide a MatchEvaluator delegate that returns the matched string after the

Answer 1

From MSDN:

Replace(String, String, Int32)   

Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.

Isn't this what you want?

Answer 2

You were probably using the static method. There is no (String, String, Int32) overload for that. Construct a regex object first and use myRegex.Replace.

Answer 3

In that case you can't use:

string str ="abc546_$defg";
str = Regex.Replace(str,"[^A-Za-z0-9]", "");

Instead you need to declare new Regex instance and use it like this:

string str ="abc546_$defg";
Regex regx = new Regex("[^A-Za-z0-9]");
str = regx.Replace(str,"",1)

Notice the 1, It represents the number of occurrences the replacement should occur.

Answer 4

Just to answer the original question... The following regex matches only the first instance of the word foo:

(?<!foo.*)foo

This regex uses the negative lookbehind (?<!) to ensure no instance of foo is found prior to the one being matched.