C# Equivalent for JS RegEx Expression

What's the C# equivalent of this JavaScript Regular Expression?

str.replace(/(\w)\w*/g, "$1");

Javascript Input + Result (Desired):

Input:  I like pie!
Result: i l p!

C# Input + Result (Using Tim's Version posted below):

Input:  I like pie!
Resul开发者_开发百科t: \1 \1 \1!

Any other ideas?

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resultString = Regex.Replace(subjectString, "([A-Z0-9_])[A-Z0-9_]*", "$1", RegexOptions.IgnoreCase);

This change is necessary because \w matches a lot more in .NET regexes than in JavaScript regexes.

(Unless you also want to match words that contain non-ASCII letters/digits, in which case `@"(\w)\w*" would be better.)

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var result = Regex.Replace(input, @"(?<x>\w)\w*", @"${x}");