{"id":1944,"date":"2023-06-02T09:37:35","date_gmt":"2023-06-02T02:37:35","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=1944"},"modified":"2023-06-02T09:38:06","modified_gmt":"2023-06-02T02:38:06","slug":"csharp-regex-lookahead","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-regular-expression\/csharp-regex-lookahead\/","title":{"rendered":"C# Regex Lookahead"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to use C# regex lookahead in the regular expression to match only if it is followed by another pattern.<\/p>\n\n\n\n

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

The regex lookahead is a feature in regular expressions that allows you to match A only if it is followed by B. The syntax of the regex lookahead is as follows:<\/p>\n\n\n

A(?=B)<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
This regex lookahead matches A only if it is followed by B.<\/p>\n\n\n\n
Suppose you have the following string:<\/p>\n\n\n
\"1 shark is 5 feet long\"<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
And you want to match the number 5 which is followed by a space and the literal string “feet” but not the number 1. To do that you can use the regex lookahead like this:<\/p>\n\n\n
\\d+(?=\\s*feet)<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
This regular expression pattern matches one or more digits but only if they are followed by zero or more whitespace characters and the word “feet”.<\/p>\n\n\n\n
Here’s the program:<\/p>\n\n\n
using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> text = \"1 shark is 5 feet long\"<\/span>;\nvar<\/span> pattern = @\"\\d+(?=\\s*feet)\"<\/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
C# Regex multiple lookaheads<\/h2>\n\n\n\n
Regular expressions support multiple lookaheads with the following syntax:<\/p>\n\n\n
A(?=B)(?=C)<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, we have two lookaheads. The A(?=B)(?=C)<\/code> regular expression matches the pattern A<\/code> if it is followed by both B<\/code> and C<\/code>.<\/p>\n\n\n\n
Here are how the regex engine processes:<\/p>\n\n\n\n
    \n
  • First, match the pattern A<\/code>.<\/li>\n\n\n\n
  • Second, evaluate the first lookahead (?=B)<\/code>  at the position immediately after A<\/code>. If it doesn’t match, the regex engine stops.<\/li>\n\n\n\n
  • Third, evaluate the second lookahead (?=C)<\/code> at the position immediately after B<\/code>. If it doesn’t match, the regex engine stops.<\/li>\n\n\n\n
  • Finally, if both tests passed, the regex engine returns the match for the pattern A<\/code>.<\/li>\n<\/ul>\n\n\n\n
    In theory, you can have as many lookaheads as you want.<\/p>\n\n\n\n
    Regex negative lookaheads<\/h2>\n\n\n\n
    Negative lookahead negates a lookahead. It matches A only if it is not followed by B:<\/p>\n\n\n
    A(?!B)<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Negative lookaheads are useful when you want to specify exclusions in your regular expression patterns. They help you find matches that do not have certain patterns following them.<\/p>\n\n\n\n
    The following example uses a negative lookahead in a regular expression to match a number that is not followed by the word “feet”:<\/p>\n\n\n
    using<\/span> System.Text.RegularExpressions;\nusing<\/span> static<\/span> System.Console;\n\n\nvar<\/span> text = \"1 shark is 5 feet long\"<\/span>;\nvar<\/span> pattern = @\"\\d+(?!\\s*feet)\"<\/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
    1<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Summary<\/h2>\n\n\n\n
      \n
    • Use negative lookahead A(?=B)<\/code> in regular expressions to match A only if it is followed by B.<\/li>\n\n\n\n
    • Use negative lookahead A(?!B)<\/code> in regular expressions to match A only if it is not<\/strong> followed by B.<\/li>\n<\/ul>\n
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