Regex to replace a prefix string-CodePudding

I want to replace a substring with a itself followed by a dot, when it is start of a longer sequence (i.e. it is a prefix). For example given the string:

"'abc' 'xyzabcdef' 'abcdef'"

I want to transform the prefix abc into abc., to give:

"'abc' 'xyzabcdef' 'abc.def'"

Note that the standalone string of abc is not transformed.

I have tried this code:

var input = @"'abc' 'xyzabcdef' 'abcdef'";
var result = Regex.Replace(input, @"\b(abc)\w ", @"$1.");
Console.WriteLine(result);

which gives:

'abc' 'xyzabcdef' 'abc.'

Note in the third output, the trailing def is also substituted. I expected the substitution of $1 would have been associated with just the group (abc), but it clearly matches the entire pattern. I'm looking for the correct formulation of the Regex.Replace() call.

CodePudding user response：

Try replacing on the regex pattern (?<=')abc(?=\w). This pattern says to match:

(?<=')  assert that single quote precedes
abc     match and consume abc
(?=\w)  assert that at least one word character follows

var input = @"'abc' 'xyzabcdef' 'abcdef'";
var result = Regex.Replace(input, @"(?<=')abc(?=\w)", @"abc.");
Console.WriteLine(result);  // 'abc' 'xyzabcdef' 'abc.def'