Regular Expressions in grep (Regex) (With Examples)

Linux Regular Expressions (Regex) are special characters that help search data based on matching complex patterns. They are used in many Linux command-line utilities, such as rename, bash, sed, grep, etc. In this guide, we’ll be focusing on the grep command.

grep Command Purpose and Syntax

grep searches for patterns in a file(s). patterns refer to one or more patterns separated by newline characters, and grep prints each line that matches a defined pattern.

# grep [operators...] patterns [file...]

Types of Regular Expressions

There are three main types of Linux regular expressions:

Basic Regular Expressions

A Basic Regular Expression (BRE) is a regular expression consisting of the exact characters of the string to be matched.

Basic Regular Expression Operators

  • . — replaces any character
  • ^ — matches start of string
  • $ — matches end of string
  • * — matches up zero or more times the preceding character
  • \ — represent special characters
  • () — groups regular expressions
  • ? — matches up exactly one character

Basic Regular Expression – Example

In this example, we list all comments in the /etc/adduser.conf file. (Note that the # sign prefaces comments in .conf files).

# grep ^# /etc/adduser.conf

Display all comments (#) in /etc/adduser.conf.
Display all comments (#) in /etc/adduser.conf.

Interval Regular Expressions

An Interval Regular Expression (IRE) is a regular expression that reports the number of characters in a string.

Interval Regular Expression Operators

  • {n} — matches the preceding character appearing ‘n’ times exactly
  • {n,m} — matches the preceding character appearing ‘n’ times but not more than m
  • {n, } — matches the preceding character only when it appears ‘n’ times or more

Interval Regular Expression – Example

In this example, we’ll find all content in the /etc/adduser.conf that contains the ‘l‘ character exactly two times consecutively.

# grep -E l{2} /etc/adduser.conf
Display content in the /etc/adduser.conf that contains the ‘l’ character exactly two times consecutively.
Display content in the /etc/adduser.conf that contains the ‘l’ character exactly two times consecutively.

Extended Regular Expressions

An Extended Regular Expression (ERE) is a regular expression that contains more than one expression.

Extended Regular Expressions Operators

The extended regular expression contains more than one expression.

  • \+  — matches one or more occurrences of the previous character
  • \? — matches zero or one occurrence of the previous character

Extended Regular Expression – Example

In this example, we’ll search /etc/adduser.conf where character ‘t‘ precedes character ‘a.’

# grep "t\+a" /etc/adduser.con
Display content in the /etc/adduser.conf where character ‘t’ precedes character ‘a.’
Display content in the /etc/adduser.conf where character ‘t’ precedes character ‘a.’

Was this article helpful?

Leave a Comment