{"id":1894,"date":"2023-05-31T17:04:22","date_gmt":"2023-05-31T10:04:22","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=1894"},"modified":"2023-05-31T20:19:25","modified_gmt":"2023-05-31T13:19:25","slug":"csharp-regex","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-regular-expression\/csharp-regex\/","title":{"rendered":"C# Regex"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to use the C# Regex<\/code> class to work with regular expressions.<\/p>\n\n\n\n

Introduction to the C# Regex class<\/h2>\n\n\n\n

The following simple program uses the Contains()<\/code> method of a String<\/code> object to check whether an input string contains the number 2:<\/p>\n\n\n

using<\/span> static<\/span> System.Console;\n\n\nvar<\/span> message = \"I have 2 apples\"<\/span>;\nvar<\/span> result = message.Contains(\"2\"<\/span>);\n\nWriteLine(result);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
If the number that you want to check is not 2 but may vary from 0 to 9, then you need to modify the program like this:<\/p>\n\n\n
var<\/span> message = \"I have 5 apples\"<\/span>;\n\nvar<\/span> result = false<\/span>;\nfor<\/span> (var<\/span> i = 0<\/span>; i <= 9<\/span>; i++)\n{\n    if<\/span> (message.Contains(i.ToString()))\n    {  \n        result = true<\/span>;\n        break<\/span>;\n    }\n}\n\nWriteLine(result);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The program sets the result<\/code> flag to false<\/code> and checks if an input string contains a single digit from 0 to 9 using a for loop. If it does, the program sets the result<\/code> to true<\/code> and breaks<\/a> the loop. Finally, the program writes the result<\/code> to the console.<\/p>\n\n\n\n
The program works as expected but it is quite verbose. To check if a string contains a single digit, you can use a regular expression.<\/p>\n\n\n\n
A regular expression describes a pattern for matching. For example, to match a single digit, you use the following pattern:<\/p>\n\n\n
\\d<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
To use this regular expression pattern in C# for matching, you will need to utilize the Regex<\/code> class.<\/p>\n\n\n\n
First, import the System.Text.RegularExpressions<\/code> namespace in the program:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a new Regex<\/code> object and pass the regular expression pattern to the Regex<\/code>‘s constructor:<\/p>\n\n\n
var<\/span> regex = new<\/span> Regex(@\"\\d\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, call the IsMatch()<\/code> method that returns true<\/code> if an input string matches the regular expression pattern:<\/p>\n\n\n
var<\/span> result = regex.IsMatch(message);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
If the input string message doesn’t match the pattern, the IsMatch()<\/code> method returns false<\/code>.<\/p>\n\n\n\n
Put it all together:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> message = \"I have 5 apples\"<\/span>;\nvar<\/span> regex = new<\/span> Regex(@\"\\d\"<\/span>);\n\nvar<\/span> result = regex.IsMatch(message);\n\nWriteLine(result);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, by using regular expressions to match, the program becomes less verbose.<\/p>\n\n\n\n
Using C# Regex.IsMatch() static method<\/h2>\n\n\n\n
Besides creating a new Regex<\/code> object and calling the IsMatch()<\/code> method, you can use the IsMatch()<\/code> static method of the Regex<\/code> class. For example:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> message = \"I have 5 apples\"<\/span>;\nvar<\/span> pattern = @\"\\d\"<\/span>;\n\nvar<\/span> result = Regex.IsMatch(message, pattern);\n\nWriteLine(result);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The IsMatch()<\/code> static method accepts two arguments, the first one is an input string and the second one is a pattern to match.<\/p>\n\n\n\n
If you have a one-time matching task, the IsMatch()<\/code> static method is more concise and convenient. However, when you need to reuse the same pattern for multiple matches, using the IsMatch()<\/code> method of a Regex<\/code> object offers better performance.<\/p>\n\n\n\n
The reason is that when you call the IsMatch()<\/code> method of a Regex<\/code> object, it compiles the pattern into an internal representation that is optimized the pattern for matching. Once compiled, the Regex<\/code> object caches the result internally so that the next match will not need to recompile the pattern again, resulting in faster matching.<\/p>\n\n\n\n
On the other hand, the IsMatch()<\/code> static method doesn’t provide the same optimizations. Instead, it compiles the regular expression pattern internally for each<\/strong> matching operation. This causes extra overhead and decreases performance if you use the same pattern for matching multiple times.<\/p>\n\n\n\n
Notice that this logic is also applied to other methods and static methods of the Regex<\/code> class like Match()<\/code> and Matches()<\/code>.<\/p>\n\n\n\n
C# Regex Match method<\/h2>\n\n\n\n
The Match()<\/code> method matches a pattern and returns the first match as a single Match<\/code> object. <\/p>\n\n\n\n
For example, the following program uses the Match()<\/code> method to output the first number found in a string:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> message = \"I have 3 apples and 5 oranges\"<\/span>;\nvar<\/span> pattern = @\"\\d\"<\/span>;\n\nvar<\/span> match = Regex.Match(message, pattern);\nWriteLine(match);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
C# Regex Matches method<\/h2>\n\n\n\n
Unlike the Match()<\/code> method, the Matches()<\/code> method returns a collection of matches found in a string. For example:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> message = \"I have 3 apples and 5 oranges\"<\/span>;\nvar<\/span> pattern = @\"\\d\"<\/span>;\n\nvar<\/span> matches = Regex.Matches(message, pattern);\n\nforeach<\/span> (var<\/span> match in<\/span> matches)\n{\n    Console.WriteLine(match);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3<\/span>\n5<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this tutorial, you learned a simple pattern that matches a single digit. In the next tutorials, you will learn how to construct more flexible patterns.<\/p>\n\n\n\n
Summary<\/h2>\n\n\n\n
    \n
  • Use Regex.IsMatch()<\/code> method to check if a regular expression pattern finds a match in a string.<\/li>\n\n\n\n
  • Use Regex.Match()<\/code> method to return the first match of a regular expression pattern in a string.<\/li>\n\n\n\n
  • Use Regex.Matches()<\/code> method to return all matches of a regular expression pattern in a string.<\/li>\n\n\n\n
  • Use methods like IsMatch()<\/code>, Match()<\/code>, and Matches()<\/code> of the Regex<\/code> object if you match a single pattern multiple times to gain higher performance. Otherwise uses the corresponding static methods of the Regex<\/code> class to make the code more concise.<\/li>\n<\/ul>\n
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