C# Regex.Replace Examples: MatchEvaluator

Regex.Replace processes text replacements.

It handles simple replacements and more complex ones. For complex pattern replacements, we use a MatchEvaluator delegate. We change a string with lowercased words to have uppercased ones.

Delegates

Example. To start, this program is meant to demonstrate the use of the Regex.Replace static method with a string replacement. You can specify a delegate of type MatchEvaluator for more complex replacements.

This program simply uses a pattern to replace all three-letter sequences starting and ending with certain letters with a replacement string. The Regex method allows you to replace variations in the string in one statement.

Based on:

.NET 4

C# program that uses Regex.Replace method

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// This is the input string we are replacing parts from.
	string input = "Dot Net Not Perls";

	// Use Regex.Replace to replace the pattern in the input.
	// ... The pattern N.t indicates three letters.
	// ... N, any character, and t.
	string output = Regex.Replace(input, "N.t", "NET");

	// Write the output.
	Console.WriteLine(input);
	Console.WriteLine(output);
    }
}

Output

Dot Net Not Perls
Dot NET NET Perls

The Regex.Replace method is a public static method and we pass it three string parameters. The first argument is the input. The second argument is the pattern. And the third argument is the replacement string data.

Next: The program replaces all parts of the source string that start with N and ending with lowercase t with another three letters.

MatchEvaluator. We look at an example of using MatchEvaluator. With Regex, you can specify a MatchEvaluator. This is a delegate method that the Regex.Replace method calls to modify the match. Here we use MatchEvaluator to uppercase matches.

Tip: You can use Regex.Replace for simple replacements by using a string argument. For complex replacements, use MatchEvaluator.

C# program that capitalizes strings

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// Input strings.
	const string s1 = "marcus aurelius";
	const string s2 = "the golden bowl";
	const string s3 = "Thomas jefferson";

	// Write output strings.
	Console.WriteLine(TextTools.UpperFirst(s1));
	Console.WriteLine(TextTools.UpperFirst(s2));
	Console.WriteLine(TextTools.UpperFirst(s3));
    }
}

public static class TextTools
{
    /// <summary>
    /// Uppercase first letters of all words in the string.
    /// </summary>
    public static string UpperFirst(string s)
    {
	return Regex.Replace(s, @"\b[a-z]\w+", delegate(Match match)
	{
	    string v = match.ToString();
	    return char.ToUpper(v[0]) + v.Substring(1);
	});
    }
}

Output

Marcus Aurelius
The Golden Bowl
Thomas Jefferson

The method TextTools.UpperFirst above is called from your code and it uses the regular expression. In Regex.Replace, we use the delegate(Match match) syntax for a private method that alters strings to have an uppercase first letter.

Tip: Delegate methods are methods you can use as variables and parameters. They introduce some syntactic complexity.

Discussion. The pattern in the Regex.Replace call uses escape sequences. The syntax of Regex.Replace's second argument is described here. The metacharacters match specific patterns (ranges) of text.

Regex pattern description

\b      Word break:
	Matches where a word starts.

[a-z]   Matches any lowercase ASCII letter.
	We only need to match words with lowercase first letters.
	This is a character range expression.

\w+     Word characters:
	Matches must have one or more characters.

When researching the problem, I found a good article at MSDN. However, the solution has some weaknesses. It isn't easy to call elsewhere in your program, and has some extra branches.

Regex.Replace Method: MSDN

What other uses does MatchEvaluator have? MSDN indicates you can use it when you need to perform validation. "You can use MatchEvaluator to perform custom verifications or operations at each Replace operation."

MatchEvaluator Delegate: MSDN

Tip: To enhance this capitalization algorithm, you could store a Dictionary of words that need special-casing, such as DeBruijn.

However: This requires a bit of manual work to find most of the names using different rules.

Summary. Regex.Replace can be used in two ways. We used a string replacement and a MatchEvaluator. The MatchEvaluator delegate offers a high degree of control. With a string, the Regex.Replace method can be used for simpler tasks.

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