C# Regex

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Regex: scrambled text in green

Regex. Patterns are everywhere.
In text,
we often discover,
and must process,
textual patterns. A regular expression describes a text-based transformation.

Regex: regular expression

Intro. The Regex class handles regular expressions. We specify patterns as string arguments. Methods (Match, Replace) are available.


Match. This program introduces the Regex class. We use its constructor and the Match method, and then handle the returned Match object.

Namespace:All these types are found in the System.Text.RegularExpressions namespace.

Pattern:The Regex uses a pattern that indicates one or more digits. The characters "55" match this pattern.

Success:The returned Match object has a bool property called Success. If it equals true, we found a match.

Based on:

.NET 4.5

Program that uses Match, Regex: C#

using System;
using System.Text.RegularExpressions;

class Program
    static void Main()
	Regex regex = new Regex(@"\d+");
	Match match = regex.Match("Dot 55 Perls");
	if (match.Success)



Method call

IsMatch, Matches. Often the Regex.Match method is useful. Match eliminates the need to search and extract strings, as with IndexOf and Substring. This makes code cleaner.

Match: GroupsMatchesMatches: QuoteIsMatch

Star:Also known as a Kleene closure in language theory. It is important to know the difference between the star and the plus.


Words:With Regex we can count words in strings. We compare this method with Microsoft Word's implementation.

Word Count

Replace. Sometimes we need to replace a pattern of text with some other text. Regex.Replace helps. We can replace patterns with a string, or with a value determined by a MatchEvaluator.

Replace:We use the Replace method, with strings and MatchEvaluators, to replace text. We replace spaces, numbers and position-based parts.

ReplaceReplace: EndReplace: NumbersReplace: Spaces

Spaces:Whitespace isn't actually white.
But it is often not needed for future processing of data.

Replace: Trim

Here:We use the simplest form of Regex.Replace. We replace all three letter words starting with C and ending in T with "cat."

Program that uses Replace: C#

using System;
using System.Text.RegularExpressions;

class Program
    static void Main()
	// ... Input string.
	string value = "I pet the cot.";

	// ... Change patterns matching "c.t" to "cat".
	string result = Regex.Replace(value, "c.t", "cat");


I pet the cat.

Split strings

Split. Do you need to extract substrings that contain only certain characters (certain digits, letters)? Split() returns a string array that will contain the matching substrings.


Numbers:We can handle certain character types, such as numbers, with the Split method. This is powerful. It handles many variations.

Split: Numbers

Caution:The Split method in Regex is more powerful than the one on the string type. But it may be slower in common cases.

String Split

Escape. This method can change a user input to a valid Regex pattern. It assumes no metacharacters were intended. The input string should be only literal characters.

Note:With Escape, we don't get out of jail free, but we do change the representation of certain characters in a string.

About part

Unescape. The term "unescape" means to do the reverse of escape. It returns character representations to a non-escaped form. This method is rarely useful.


Files. We often need to process text files.
The Regex type,
and its methods,
are used for this. But we need to combine a file input type, like StreamReader, with the Regex code.

StreamReader:Here we use StreamReader to stream a file in line-by-line. Then we process each line with the Regex class.

Regex: Files
Hypertext markup language: HTML

HTML. Regex can be used to process or extract parts of HTML strings. There are problems with this approach. But it works in many situations.

Title, P:We focus on title and P elements.
These are common tags in HTML pages.

Title: HTMLParagraphs: HTML

Remove HTML:We also remove all HTML tags.
Please be cautious with this article.
It does not work on many HTML pages.

Remove HTML Tags
Dots: colored circles

RegexOptions. With the Regex type, the RegexOptions enum is used to modify method behavior. Often I find the IgnoreCase value helpful.

IgnoreCase:Lowercase and uppercase letters are distinct in the Regex text language. IgnoreCase changes this.


Multiline:We can change how the Regex type acts upon newlines with the RegexOptions enum. This is often useful.

Program that uses RegexOptions.IgnoreCase: C#

using System;
using System.Text.RegularExpressions;

class Program
    static void Main()
	const string value = "TEST";
	// ... This ignores the case of the "TE" characters.
	if (Regex.IsMatch(value, "te..", RegexOptions.IgnoreCase))



Performance optimization

Is Regex fast? This question is a topic of great worldwide concern. Sadly Regex often results in slower code than imperative loops. But we can optimize Regex usage.

1. Compile.Using the RegexOptions.Compiled argument to a Regex instance will make it execute faster. This however has a startup penalty.

RegexOptions.CompiledRegex Performance

2. Replace with loop.Some Regex method calls can be replaced with a loop.
The loop is much faster.

Regex vs. Loop

3. Use static fields.You can cache a Regex instance as a static field.
This often improves performance.

Static Regex
Algorithm: shuffle elements

Research. A regular expression can describe any "regular" language. These languages are ones where complexity is finite: there is a limited number of possibilities.

Caution:Some languages, like HTML, are not regular languages. This means you cannot fully parse them with traditional regular expressions.

Automaton:A regular expression is based on finite state machines. These automata encode states and possible transitions to new states.

Operator keyword

Operators. Regular expressions use compiler theory. With a compiler, we transform regular languages (like Regex) into tiny programs that mess with text.

These expressions are commonly used to describe patterns. Regular expressions are built from single characters, using union, concatenation, and the Kleene closure, or any-number-of, operator. Aho et al., p. 187

A summary. Regular expressions are a concise way to process text data. This comes at a cost. For performance, we can rewrite Regex calls with low-level char methods.

Representations. Regex is a high-level representation of the same logic expressed with loops and char arrays. This logic is represented in a simple, clear way.