Regular Expressions (Regex)
Regex patterns match and filter text. In Linux they are commonly used with grep, sed, and awk.
Overview
- Regex is compact and composable
- Start with simple patterns and add anchors or grouping only when needed
- Most mistakes come from precedence, escaping, or incorrect anchoring
Reference
Core constructs
| Pattern |
Meaning |
. |
Any character |
* |
Zero or more of the previous token |
+ |
One or more of the previous token |
? |
Zero or one of the previous token |
^ |
Start of line |
$ |
End of line |
\b |
Word boundary |
\w |
Word character |
[] |
Character class |
() |
Grouping |
| |
OR operator |
grep modes
| Option |
Meaning |
grep |
Basic regex |
grep -E |
Extended regex; usually preferred |
grep -v |
Invert the match |
Common patterns
| Use case |
Pattern |
| Starts with a word |
^word |
| Ends with a word |
word$ |
| Contains a word |
word |
| Word boundary |
\bword\b |
| Starts with A, ends with B |
^A.*B$ |
| A or B |
A|B |
Examples
| Task |
Command |
| Exclude comment lines |
grep -v '#' /etc/ssh/sshd_config |
Match words starting with Permit |
grep -E '\bPermit\w*' /etc/ssh/sshd_config |
Match words ending with Authentication |
grep -E '\w*Authentication\b' /etc/ssh/sshd_config |
Match lines containing Key |
grep 'Key' /etc/ssh/sshd_config |
Match lines starting with Password and containing yes |
grep -E '^Password.*yes' /etc/ssh/sshd_config |
Match lines ending with yes |
grep -E 'yes$' /etc/ssh/sshd_config |
Best practices
- Prefer
grep -E for extended regex
- Avoid unnecessary
cat; pipe the file directly into the command
- Build patterns incrementally and verify anchors and grouping
Mental Model
- Regex matches text by combining anchors, quantifiers, and grouping
^ and $ constrain position* and + control repetition- Parentheses and
| change matching logic