Regular Expressions in Linux

Regular expressions provide powerful methods for searching and parsing text. In Linux, they’re used extensively in commands like grep, find, locate, less, and editors like vim. This guide focuses on POSIX-compliant regex syntax: Basic (BRE) and Extended (ERE).

What Are Regular Expressions?

Regexes define symbolic patterns for matching text. While similar to shell wildcards, they offer more precision and versatility across programming and shell tools.

grep: The Regex Workhorse

grep searches lines that match a given regex:

grep -h bzip dirlist*.txt

Option	Description
-i	Ignore case
-v	Invert match
-c	Count matches
-l / -L	Show matching / non-matching file names
-n	Show line numbers
-h	Suppress filename in output

Metacharacters and Anchors

Metacharacters include ^ $ . [ ] { } * + ? ( ) |. Quote expressions to avoid shell interference:

grep -h '.zip' dirlist*.txt

• .: Matches any character
• ^: Start of line
• $: End of line

Bracket Expressions and Character Classes

grep '[bg]zip' dirlist*.txt

grep '[^bg]zip' dirlist*.txt

Class	Matches
[:alnum:]	Letters and digits
[:digit:]	Digits only
[:lower:], [:upper:]	Lower / Uppercase letters
[:space:]	Whitespace

Ensure consistent behavior by exporting ASCII locale:

export LANG=POSIX

BRE vs ERE

• BRE: Requires escaping of ( ) { }
• ERE: Supports alternation |, quantifiers, groups

grep -E 'AAA|BBB|CCC' file.txt

Quantifiers

Syntax	Meaning
?	Zero or one
*	Zero or more
+	One or more
{n}	Exactly n
{n,m}	Between n and m
{n,}	At least n

Practical Examples

Validate Phone Numbers

grep -Ev '^\([0-9]{3}\) [0-9]{3}-[0-9]{4}$' phonelist.txt

Find Ugly Filenames

find . -regex '.*[^-_./0-9a-zA-Z].*'

Regex Search with locate

locate --regex 'bin/(bz|gz|zip)'

Regex Search in less

less phonelist.txt
/^\([0-9]{3}\) [0-9]{3}-[0-9]{4}$

Regex in vim

/[0-9]\{3}\) [0-9]\{3\}-[0-9]\{4\}

Enable highlighting with :set hlsearch

Find Regex-Supporting Tools

cd /usr/share/man/man1
zgrep -El 'regex|regular expression' *.gz

Conclusion

Regex enables powerful text matching across Linux tools. By mastering grep, find, locate, and text editors, users can filter, validate, and manipulate data with precision. Understanding POSIX BRE and ERE unlocks efficient workflows in scripting, log analysis, and system administration.