How to use grep with regular expressions

Match complex text patterns using basic and extended regular expressions with grep's -E and -P flags.

Step-by-Step: Use grep with Regular Expressions
1. Match Multiple Alternatives with grep Extended Regex
2. Match a Pattern at the Start or End of a Line
3. Match IP Addresses with grep
4. Use Perl-Compatible Regex with grep
Common Issues

Match text patterns using regular expressions in grep, including character classes, quantifiers, alternation, and anchors with the -E (extended) and -P (Perl) flags.

Step-by-Step: Use grep with Regular Expressions

1. Match Multiple Alternatives with grep Extended Regex

grep -E "error|warning|critical" /var/log/syslog

The grep -E flag enables Extended Regular Expressions (ERE), which supports | (alternation), + (one or more), and ? (zero or one) without backslash escaping.

2. Match a Pattern at the Start or End of a Line

grep "^server" /etc/nginx/nginx.conf
grep "\.log$" /etc/logrotate.d/nginx

The ^ anchor matches the start of a line. The $ anchor matches the end. These are supported in both BRE and ERE modes.

3. Match IP Addresses with grep

grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" /var/log/auth.log

4. Use Perl-Compatible Regex with grep

grep -P "\d{4}-\d{2}-\d{2}" /var/log/app.log

The grep -P flag enables Perl-Compatible Regular Expressions (PCRE), which adds \d (digit), \w (word character), lookahead, and lookbehind.

Common Issues

"Invalid regular expression"— Special characters like (, ), {, }, |, and + must be escaped in Basic Regular Expression (BRE) mode. Use -E for extended regex where these characters work without escaping. See grep: Invalid regular expression.