Regular expression find and replace to wrap tags

Question

Does anyone know how to use regex to find and replace certain word with

[Keyword]

I tried to use Regex.Replace() but it

Answer 1

You do not need any lambda to re-use the whole match value inside a replacement pattern, you may refer to the whole match value using the $& backreference. See Substitutions in Regular Expressions:

$&    Includes a copy of the entire match in the replacement string. For more information, see Substituting the Entire Match.

C# code:

var input = "Hello World!";
var keyword = "Hello";
var result = Regex.Replace(input, Regex.Escape(keyword), "<b>$&</b>");
Console.WriteLine(result);

See the online C# demo.

However, this code above makes little sense as you can achieve the same with a mere string .Replace: var result = input.Replace(keyword, $"<b>{keyword}</b>"). Hence, here are some ideas where you want to use the regex:

Whole word matching

• Regular word boundaries (if keyword consists of letters/digits/underscores ("word" chars) only): Regex.Replace(input, $@"\b{keyword}\b", "<b>$&</b>")
• Unambiguous word boundaries (if keyword may contain non-word chars): Regex.Replace(input, $@"(?<!\w){Regex.Escape(keyword)}(?!\w)", "<b>$&</b>")
• Whitespace word boundaries (a whole word is a chunk of non-whitespace chars wrapped with whitespaces or start/end of string): Regex.Replace(input, $@"(?<!\S){Regex.Escape(keyword)}(?!\S)", "<b>$&</b>")

Case insensitive matching

You may need a regex like this to ensure the same case of the keyword in the replaced string when performing a case insensitive regex search:

Regex.Replace(input, Regex.Escape(keyword), "<b>$&</b>", RegexOptions.IgnoreCase)

See another C# demo:

var keyword = "A+";
Console.WriteLine("Unambiguous WB: " + Regex.Replace("A+B and A++", $@"(?<!\w){Regex.Escape(keyword)}(?!\w)", "<b>$&</b>"));
// => Unambiguous WB: A+B and <b>A+</b>+
keyword = "Hello";
Console.WriteLine("Regular WB: " + Regex.Replace("Hello World! Hello,World!", $@"\b{keyword}\b", "<b>$&</b>"));
// => Regular WB: <b>Hello</b> World! <b>Hello</b>,World!
Console.WriteLine("Whitespace WB: " + Regex.Replace("Hello, Hello Hello!", $@"(?<!\S){Regex.Escape(keyword)}(?!\S)", "<b>$&</b>"));
// => Whitespace WB: Hello, <b>Hello</b> Hello!
keyword = "hello";
Console.WriteLine("Case innsensitive: " + Regex.Replace("Hello, hello World!", Regex.Escape(keyword), "<b>$&</b>", RegexOptions.IgnoreCase));
// => Case innsensitive: <b>Hello</b>, <b>hello</b> World!

Answer 2

You may try this:

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
        string
            input = "Hello World!",
            keyword = "Hello";

        var result = Regex
            .Replace(input, keyword, m => 
                String.Format("<b>{0}</b>", m.Value));
        Console.WriteLine(result);
    }
}