C# String replace with dictionary and Regex condition

Question

I want to replace values of a string with a dictionary that I have but only if they comply a regex condition.

This is what I have:

string input = @"A.4 AND ([10] A.4 OR A.4) OR [10]A.4 A.5 [10]A.5";
Dictionary <string, string> dict = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase) {
                                                    {"A.4", "Test"},
                                                    {"A.5", "Test2"},
                                                    };
var output = dict.Aggregate(input, (current, value) => current.Replace(value.Key, value.Value));

//current output = "Test AND ([10] Test OR Test) OR [10]Test Test2 [10]Test2"
//wished output = "Test AND ([10] A.4 OR Test) OR [10]A.4 Test2 [10]A.5"

I don't want to replace the text when there is "[10]" or "[10] " in front of the text. I think that I should use a regex or something similar but I don't know how.

You already mentioned a Regex. Have you tried using Regex.Replace instead of this? Or at least write a Regex that matches the parts you want? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Oct 5, 2022 at 9:34
What do the tags look like? If they have a pattern you could use it in a single regex, eg (?<!\[10\]\s?)A\.\d+ would match all A.n values without the prefix — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Oct 5, 2022 at 9:38

Mathias R. Jessen · Accepted Answer · 2022-10-05 09:43:29Z

2

You could use regex to perform the replace operation:

var output = dict.Aggregate(input, (current, sub) => Regex.Replace(current, $@"(?<!\[10\]\s?){Regex.Escape(sub.Key)}", sub.Value));

The negative lookbehind assertion (?<!\[10\]\s?) will ensure the matched term never follows [10] or [10] .

As Panagiotis Kanavos notes you can skip the Aggregate call completely by passing a delegate that performs a dictionary lookup to Regex.Replace:

var replacePattern = $@"(?<!\[10\]\s?)(?:{string.Join('|', dict.Keys.Select(Regex.Escape))})";
var output = Regex.Replace(input, replacePattern, (m) =>  dict[m.Value]);

edited Oct 5, 2022 at 9:43

answered Oct 5, 2022 at 9:35

Mathias R. Jessen

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4 Comments

Panagiotis Kanavos Over a year ago

There's no need for Aggregate, at least for replacement. Regex.Replace has an overload with a delegate that returns the replacement value.

Mathias R. Jessen Over a year ago

@PanagiotisKanavos Good point, added to answer

Evil Morty Over a year ago

@Mathias R. Jessen Thank you so much, that totally worth for me. However I will use the solution with Aggregate because I have keys like (A.4 and A.41) and with the second solution the A.41 is not replaced with the correct value, it's replace with the A.4 value and then there is a 1 at the end. So I'm using the first one with dict.Reverse().Aggregate(... being dict a SortedDictionary so the longest keys will be checked first.

Mathias R. Jessen Over a year ago

@EvilMorty add \b after the last ) in replacePattern to fix that :)

Panagiotis Kanavos · Accepted Answer · 2022-10-05 09:44:19Z

1

Regex.Replace has an overload with a delegate that creates replacement values. If the tags have a pattern, a single regular expression can be used to match them and replace them using the dictionary values.

This regex matches tags in the form A.n where n a number:

var regex=new Regex(@"(?<!\[10\]\s?)A\.\d+");

var output=regex.Replace(input,match=>dict[match.Value]);

Console.WriteLine(output);

This produces

Test AND ([10] A.4 OR Test) OR [10]A.4 Test2 [10]A.5

(?<!...) is a negative lookbehind pattern. It matches strings that don't start with the negated pattern. (?<!\[10\]\s?) matches strings that don't start with [10] and an optional space.

answered Oct 5, 2022 at 9:44

Panagiotis Kanavos

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