C# Regex Replacing Template content

Question

While using Regex to replace keywords with value in a template, I tested the following code.

string input = "Welcome {{friend}} Get my new {{id}} with {{anonymous}} People";
            Dictionary<string, string> mydict = new Dictionary<string, string> ();
            mydict.Add("friend", "<<My Friend>>");
            mydict.Add("id", "<<Your ID>>");
            string pattern = @"(?<=\{{2})[^}}]*(?=\}{2})";// @"\{{2}^(.*?)$\}{2}";//"^[{{\\w}}]$";
            //var m = Regex.Match(input, @"\{{(.*)\}}");
            string regex = Regex.Replace(input, pattern, delegate(Match match) {
                string v = match.ToString();
                return mydict.ContainsKey(v) ? mydict[v] : v;

            });

            Console.WriteLine(regex);

The curley braces still remain in the output which is not desired

I need <<My Friend>> instead of {{ <<My Friend>> }}. I would appreciate your suggestion.

For some alternatives: What's a good way of doing string templating in .NET? — Fildor
– Fildor, Commented Mar 29, 2018 at 13:02
@Fildor That's one of these "best practices" questions from 2009 that would have been closed as too broad had it been asked today. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Mar 29, 2018 at 13:06
@dasblinkenlight Yes, but nevertheless, there are some alternatives in the answers that may be worth consideration. — Fildor
– Fildor, Commented Mar 29, 2018 at 13:10

Wiktor Stribiżew · Accepted Answer · 2018-03-29 12:58:24Z

3

You may use a simple {{(.*?)}} regex and use the Group 1 vlaue to check for the dictionary match:

string pattern = @"{{(.*?)}}";
string regex = Regex.Replace(input, pattern, delegate(Match match) {
     string v = match.Groups[1].Value;
     return mydict.ContainsKey(v) ? mydict[v] : v;
});
// => Welcome <<My Friend>> Get my new <<Your ID>> with anonymous People

The same code with a lambda expression:

string regex = Regex.Replace(input, pattern, x =>
     mydict.ContainsKey(match.Groups[1].Value) ?
                mydict[match.Groups[1].Value] : match.Groups[1].Value;
});

See the C# demo.

Note that [^}}] does not mean match any text other than }}, it just matches any char other than }, same as [^}], so .*? is preferable in this case. Or even \w+ if you only have letters, digits and underscores in between {{ and }}.

answered Mar 29, 2018 at 12:58

Wiktor Stribiżew

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Sergey Kalinichenko · Accepted Answer · 2018-03-29 13:41:51Z

2

Braces remain in the original text because you are using zero-width lookahead and lookbehind constructs. This leaves the content matched by (?<=...) and (?=...) outside regex's captured value, so it does not get replaced.

To fix this problem remove lookahead and lookbehind from your regex, put a capturing group around the text of the tag, and use it to search replacement dictionary:

string pattern = @"\{{2}([^}}]*)\}{2}";
...
var v = match.Group[1].Value;
return mydict.ContainsKey(v) ? mydict[v] : v;

edited Mar 29, 2018 at 13:41

answered Mar 29, 2018 at 12:57

Sergey Kalinichenko

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2 Comments

Sergey Kalinichenko Over a year ago

A downvote of the question and a correct solution without as much as a comment is quite obnoxious. You should at least say what you think is wrong.

Fildor Over a year ago

Hm, From Wiktor's answer: "Note that [^}}] does not mean match any text other than }}, it just matches any char other than }, same as [^}]" - but that doesn't deserve a dv, does it? It still works.

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