{"id":1953,"date":"2023-06-02T12:02:43","date_gmt":"2023-06-02T05:02:43","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=1953"},"modified":"2023-06-02T12:03:09","modified_gmt":"2023-06-02T05:03:09","slug":"csharp-regex-backreference","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-regular-expression\/csharp-regex-backreference\/","title":{"rendered":"C# Regex Backreference"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn about C# regex backreference to reference back to a capturing group within the same regular expression.<\/p>\n\n\n\n

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

Backreference allows you to reference back to a captured group within the same regular expression. Backrefence is useful for matching repeated occurrences of a pattern. <\/p>\n\n\n\n

To specify a backreference, you use \\n<\/code> where n represents the number of a captured group that you want to reference. Note that the first captured group is \\1<\/code> , the second capture group is \\2<\/code>, and so on.<\/p>\n\n\n\n

The following shows an example of using a backreference that matched a duplicate word in a sentence: <\/p>\n\n\n

(\\w+)\\s+\\1<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
For example, this pattern will match the words “the the” in the sentence:<\/p>\n\n\n\n
“it’s the the best”<\/p>\n\n\n\n
Here’s the explanation of the pattern:<\/p>\n\n\n\n
    \n
  • (\\w+): captures one or more word characters and stores them in the first capturing group.<\/li>\n\n\n\n
  • \\s+: matches one or more whitespace characters<\/li>\n\n\n\n
  • \\1: refers to the text captured by the first capturing group.<\/li>\n<\/ul>\n\n\n\n
    The following program returns the duplicate word “the” in the sentence “it’s the the best”:<\/p>\n\n\n
    using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> text = \"It's the the best\"<\/span>;\nvar<\/span> pattern = @\"(\\w+)\\s+\\1\"<\/span>;\n\nvar<\/span> matches = Regex.Matches(text, pattern);\n\nforeach<\/span> (var<\/span> match in<\/span> matches)\n{\n    WriteLine(match);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Output:<\/p>\n\n\n
    the the<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    The following program removes the repeated word using the backreference:<\/p>\n\n\n
    using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> text = \"It's the the best\"<\/span>;\nvar<\/span> pattern = @\"(\\b\\w+)\\s+\\b\\1\\b\"<\/span>;\n\n\nvar<\/span> result = Regex.Replace(text, pattern, \"$1\"<\/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
    It's the best<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Notice that we use the Replace()<\/code> method to replace the pattern with the “$1<\/code>” in the text. The “$1<\/code>” specifies the replacement pattern, which is the captured word ($1<\/code>).<\/p>\n\n\n\n
    Regex backreferences are useful in some situations, for example:<\/p>\n\n\n\n
      \n
    • Finding duplicate words or phrases in sentences (already demonstrated earlier).<\/li>\n\n\n\n
    • Matching repeated patterns.<\/li>\n\n\n\n
    • Ensuring symmetry in string structures.<\/li>\n\n\n\n
    • Validating input based on previous matches.<\/li>\n<\/ul>\n\n\n\n
      Summary<\/h2>\n\n\n\n
        \n
      • Use a backreference to reference back to a captured group within the same regular expression. <\/li>\n<\/ul>\n
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