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Unix Concepts   «Prev  Next»
Lesson 2 What is a regular expression?
Objective Describe regular expressions.

Define a Regular Expression in Unix

You should already be familiar with the grep command, which has this general form:
% grep pattern file

grep searches one or more files for the specified pattern and displays all lines containing that pattern. The pattern argument is a regular expression. Using regular expressions to perform searches often is called pattern matching .
In the simplest cases, a regular expression is a literal string or sequence of characters. For example, bat is a regular expression that describes the literal characters b, a, and t, strung together. It describes a pattern found in words like bat, bath, or acrobat.
Regular expressions also can include special characters that let you perform “wildcard searches.” For example, the regular expression b[ae]t describes a pattern such as bat or bet. The brackets ([ ]) are an example of metacharacters, also called regular expression syntax.
Don’t confuse regular expressions with the wildcard patterns used to match file names. File-matching wildcards are used by the shell to match the names of files. By contrast, regular expressions are used by programs to search the contents of files. Some common programs that use regular expression syntax are grep, more, and vi. Beginners often are confused because some special characters, such as * and [ ], are used both as regular expression metacharacters and as file name wildcards.

  1. regular expression: A regular expression describes a pattern using literal characters and optional metacharacters known as regular expression syntax.
  2. pattern matching: Pattern matching is the task of using regular expressions to search for text.
  3. string: A string is a sequence of characters.
  4. metacharacter: A metacharacter is a character with special meaning in regular expressions and is not treated literally. Examples include the * and . metacharacters.

In the next lesson, you will learn how quotes affect the shell’s interpretation of regular expressions.