C# 9.0 in a Nutshell

Chapter 25. Regular Expressions

The regular expressions language identifies character patterns. The .NET types supporting regular expressions are based on Perl 5 regular expressions and support both search and search/replace functionality.

Regular expressions are used for tasks such as:

• Validating text input such as passwords and phone numbers

• Parsing textual data into more structured forms (e.g., a NuGet version string)

• Replacing patterns of text in a document (e.g., whole words only)

This chapter is split into both conceptual sections teaching the basics of regular expressions in .NET, and reference sections describing the regular expressions language.

All regular expression types are defined in System.Text.RegularExpressions.

Note

The samples in this chapter are all preloaded into LINQPad, which also includes an interactive RegEx tool (press Ctrl+Shift+F1). An online tool is available at http://regexstorm.net/tester.

Regular Expression Basics

One of the most common regular expression operators is a quantifier. ? is a quantifier that matches the preceding item 0 or 1 time. In other words, ? means optional. An item is either a single character or a complex structure of characters in square brackets. For example, the regular expression "colou?r" matches color and colour, but not colouur:

Console.WriteLine (Regex.Match ("color",   @"colou?r").Success);  // True
Console.WriteLine (Regex.Match ("colour",  @"colou?r").Success); // True Console.WriteLine (Regex.Match ("colouur", ...