Extended Regular Expressions

Extended Regular Expressions (ERE) let you write more expressive patterns without backslash escapes for common operators. In this guide, you’ll learn how to use quantifiers, character classes, grouping, alternation, and negation with GNU grep (and similar tools) to craft powerful searches.

Using grep -E vs egrep

Both grep -E and egrep enable ERE syntax, so you don’t need to escape +, ?, {}, or |:

Command	Description
`grep -Er '0+' /etc/`	Recursively search for one or more `0` in `/etc/`.
`egrep -r '0+' /etc/`	Same as above using `egrep`.

$ grep -Er '0+' /etc/
$ egrep -r '0+' /etc/

Note

Under the hood, egrep is equivalent to grep -E. Future versions of GNU grep may deprecate egrep.

Curly-Brace Quantifiers

Curly braces let you specify exact or range-based repetition counts:

Syntax	Meaning
`{n}`	Exactly n occurrences
`{n,}`	At least n occurrences
`{,m}`	At most m occurrences
`{n,m}`	Between n and m occurrences

Examples

# At least three zeros
$ egrep -r '0{3,}' /etc/

# One followed by up to three zeros (matches "1" as well)
$ egrep -r '10{,3}' /etc/

# Exactly three zeros
$ egrep -r '0{3}' /etc/

`?` and `*` Quantifiers

x? – zero or one x
x* – zero or more x

To match both disable and disabled, make the final d optional:

$ egrep -r 'disable(d)?' /etc/

Character Classes and Ranges

Define a set or range of characters using square brackets:

[abc] matches a, b, or c
[a-z] matches any lowercase letter
[0-9] matches any digit

The image shows a dark-themed interface with a command line prompt and examples of character ranges or sets, such as `[a-z]` and `[0-9]`. The text "KodeKloud" is visible in the corner.

Match `cat` or `cut`

$ egrep -r 'c[au]t' /etc/

Matching Device Files under `/dev`

A simple '/dev/.*' pattern is too greedy:

$ egrep -r '/dev/.*' /etc/

To restrict matches to letters plus an optional digit:

$ egrep -r '/dev/[a-z]+[0-9]?' /etc/

Warning

If device names include uppercase letters or multiple segments (e.g., /dev/tty0p0), use grouping and repetition to cover all cases.

$ egrep -r '/dev/([a-zA-Z]+[0-9]?)+'

Sub-Expressions and Grouping

Parentheses () treat a group of tokens as a single unit. In arithmetic:

The image shows a dark interface with a command line prompt on the left and a calculation on the right, demonstrating the expression "1+2*3" which equals "7".

1 + 2 * 3 = 1 + (2×3) = 7
(1 + 2) * 3 = 3×3 = 9

In regex:

# Repeat a letter+digit group one or more times
$ egrep -r '/dev/([a-zA-Z]+[0-9]?)+'

Alternation with `|`

Use | to match one pattern or another.

# Match "enabled" or "disabled"
$ egrep -r 'enabled|disabled' /etc/

# Case-insensitive mix with optional "d"
$ egrep -ir 'enable(d)?|disable(d)?' /etc/

Negated Character Classes

Prefix a class with ^ to invert it:

# "http" not followed by "s"
$ egrep -r 'http[^s]' /etc/

# Slash not followed by a lowercase letter
$ egrep -r '/[^a-z]' /etc/

Beyond grep: Other Regex Tools

Most Linux utilities support ERE or similar syntax:

Tool	Use Case
`sed`	Stream editing
`awk`	Field-based text processing
Text editors	Interactive search & replace

For interactive testing, try Regexr.

References

Happy pattern crafting!

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