Question

I’m working with an array of strings, and would like to do the following:

//Regex regex; List<string> strList; List<string> strList2; 
foreach (string str in strList){
    if (regex.IsMatch(str)) {      //only need in new array if matches...
        strList2.Add(regex.Replace(str, myMatchEvaluator)) 
                                   //but still have to apply transformation
    }
}

Now, I know that works, but that effectively means running the same regex twice on each string in the array. Is there a way to collapse both of these steps – the filtering and the transformation – into one regex-parsing call?

(One that would work most of the time is

string str2 = regex.Replace(str, myMatchEvaluator);
if (str2 == str)
    strList2.Add(str2);

But that would often throw out some valid matches that still didn’t need replacement.)

EDIT: A regex example, roughly similar to mine, to illustrate why this is tricky:
Imagine looking for words at the beginning of lines in a log file, and wanting to capitalize them.

The regex would be new Regex("^[a-z]+", RegexOptions.IgnorePatternWhiteSpace), and the replacement function would be match => match.ToUpper().

Now some first words are already capitalized, and I don’t want to throw them away. On the other hand, I don’t want to upper-case all instances of the word on the line, just the first one.

Answer 1

Building off of all of the answers I’ve received, the following works:

void AddToIfMatch(List<string> list, string str; Regex regex; 
                                        MatchEvaluator evaluator)
{
    bool hasBeenEvaluated = false;
    string str2 = regex.Replace(
        str, 
        m => {HasBeenEvaluated = true; return evaluator(m);}
    );
    if( hasBeenEvaluated ) {list.Add(str2);}
}