C# Regex.Match

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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

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


Question

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)
	{
	    Console.WriteLine(match.Value);
	}
    }
}

Output

55

Example

Static method. Here we match parts of a string (a file name in a directory path). We only accept ranges of characters and some punctuation. On Success, we access the group.

Static:We use the Regex.Match static method. It is also possible to call Match upon a Regex object.

Success:We test the result of Match with the Success property. When true, a Match occurred and we can access its Value or Groups.

Groups:This collection is indexed at 1, not zero—the first group is found at index 1. This is important to remember.

Groups
Program that uses Regex.Match: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// First we see the input string.
	string input = "/content/alternate-1.aspx";

	// Here we call Regex.Match.
	Match match = Regex.Match(input, @"content/([A-Za-z0-9\-]+)\.aspx$",
	    RegexOptions.IgnoreCase);

	// Here we check the Match instance.
	if (match.Success)
	{
	    // Finally, we get the Group value and display it.
	    string key = match.Groups[1].Value;
	    Console.WriteLine(key);
	}
    }
}

Output

alternate-1

Pattern details

@"              This starts a verbatim string literal.
content/        The group must follow this string.
[A-Za-z0-9\-]+  One or more alphanumeric characters.
(...)           A separate group.
\.aspx          This must come after the group.
$               Matches the end of the string.

Continue

NextMatch. More than one match may be found. We can call the NextMatch method to search for a match that comes after the current one in the text. NextMatch can be used in a loop.

Here:We match all the digits in the input string (4 and 5). Two matches occur, so we use NextMatch to get the second one.

Return:NextMatch returns another Match object—it does not modify the current one. We assign a variable to it.

Program that uses NextMatch: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	string value = "4 AND 5";

	// Get first match.
	Match match = Regex.Match(value, @"\d");
	if (match.Success)
	{
	    Console.WriteLine(match.Value);
	}

	// Get second match.
	match = match.NextMatch();
	if (match.Success)
	{
	    Console.WriteLine(match.Value);
	}
    }
}

Output

4
5

String type

Preprocess. Sometimes we can preprocess strings before using Match() on them. This can be faster and clearer. Experiment. I found using ToLower to normalize chars was a good choice.

ToLower
Program that uses ToLower, Match: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// This is the input string.
	string input = "/content/alternate-1.aspx";

	// Here we lowercase our input first.
	input = input.ToLower();
	Match match = Regex.Match(input, @"content/([A-Za-z0-9\-]+)\.aspx$");
    }
}

Static

Static. Often a Regex instance object is faster than the static Regex.Match. For performance, we should usually use an instance object. It can be shared throughout an entire project.

Static Regex

Sometimes:We only need to call Match once in a program's execution. A Regex object does not help here.

Class:Here a static class stores an instance Regex that can be used project-wide. We initialize it inline.

Static Class
Program that uses static Regex: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// The input string again.
	string input = "/content/alternate-1.aspx";

	// This calls the static method specified.
	Console.WriteLine(RegexUtil.MatchKey(input));
    }
}

static class RegexUtil
{
    static Regex _regex = new Regex(@"/content/([a-z0-9\-]+)\.aspx$");
    /// <summary>
    /// This returns the key that is matched within the input.
    /// </summary>
    static public string MatchKey(string input)
    {
	Match match = _regex.Match(input.ToLower());
	if (match.Success)
	{
	    return match.Groups[1].Value;
	}
	else
	{
	    return null;
	}
    }
}

Output

alternate-1

Pound symbol

Numbers. A common requirement is extracting a number from a string. We can do this with Regex.Match. To get further numbers, consider Matches() or NextMatch.

Digits:We extract a group of digit characters and access the Value string representation of that number.

Parse:To parse the number, use int.Parse or int.TryParse on the Value here. This will convert it to an int.

int.Parseint.TryParse
Program that matches numbers: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	// ... Input string.
	string input = "Dot Net 100 Perls";

	// ... One or more digits.
	Match m = Regex.Match(input, @"\d+");

	// ... Write value.
	Console.WriteLine(m.Value);
    }
}

Output

100

Property

Value, length, index. A Match object, returned by Regex.Match has a Value, Length and Index. These describe the matched text (a substring of the input).

Value:This is the matched text, represented as a separate string. This is a substring of the original input.

Length:This is the length of the Value string.
Here, the Length of "Axxxxy" is 6.

Index:The index where the matched text begins within the input string. The character "A" starts at index 4 here.

Program that shows value, length, index: C#

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
	Match m = Regex.Match("123 Axxxxy", @"A.*y");
	if (m.Success)
	{
	    Console.WriteLine("Value  = " + m.Value);
	    Console.WriteLine("Length = " + m.Length);
	    Console.WriteLine("Index  = " + m.Index);
	}
    }
}

Output

Value  = Axxxxy
Length = 6
Index  = 4

Method

IsMatch, Matches. In the .NET Framework, there are some other matching methods. Matches() returns multiple Match objects at once. And IsMatch tells us whether a match exists.

MatchesMatches: QuoteIsMatch

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

Star

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

Word Count
String

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
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.

Split

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
Semicolon

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.

Escape
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.

Unescape
File

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.

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.

IgnoreCase

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

Multiline
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))
	{
	    Console.WriteLine(true);
	}
    }
}

Output

True

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—an example is provided above.


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.

C#