c# - Check if String Contains Number from List, Remove that Number from String

Question

I want to check if a string contains a word or number from a list and remove it from the string.

I want to do this for multiple matches found.

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The sentence reads

This is a 01 02 03 (01) (02) (03) no01 no02 no03 test

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I need the Regex.Replace to remove only the full 01, 02, 03, not the ones inside other words.

This is a (01) (02) (03) no01 no02 no03 test

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But it only removes the occurrences of 03, the last item in the match list, in all places.

This is a 01 02 (01) (02) () no01 no02 no test

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http://rextester.com/BCEXTJ37204

C#

List<string> filters = new List<string>();
List<string> matches = new List<string>();

string sentence = "This is a 01 02 03 (01) (02) (03) no01 no02 no03 test";
string newSentence = string.Empty;

// Create Filters List
for (int i = 0; i < 101; i++)
{
    filters.Add(string.Format("{0:00}", i)); // 01-100
}

// Find Matches
for (int i = 0; i < filters.Count; i++)
{
    // Add to Matches List
    if (sentence.Contains(filters[i]))
    {
        matches.Add(filters[i]); // will be 01, 02, 03
    }
}

// Filter Sentence
for (int i = 0; i < matches.Count; i++)
{
    newSentence = Regex.Replace(sentence, matches[i], "", RegexOptions.IgnoreCase);
}

// Display New Sentence
Console.WriteLine(newSentence);

I tried changing string.Format() to @"{0:00}" to match whole words but it doesn't work.

Answer 1

The problem is that you are invoking your regex matcher repeatedly on the original string. That's why only the last change "sticks", while the rest get discarded:

newSentence = Regex.Replace(sentence, matches[i], "", RegexOptions.IgnoreCase);

If you change this to call Replace on newSentence, it is going to work correctly:

newSentence = sentence;
for (int i = 0; i < matches.Count; i++) {
    newSentence = Regex.Replace(newSentence, matches[i], "", RegexOptions.IgnoreCase);
}

However, this is suboptimal: you would be better off concatenating all replacements into a single regex, like this:

newSentence = Regex.Replace(
    sentence
,   @"(?<=s|^)(" + string.Join("|", matches) + @")(?=s|$)"
,   ""
,   RegexOptions.IgnoreCase
);

You can also remove pre-checks of filters that constructs matches, because regex engine would take care of it pretty efficiently.

Demo.