Basis

Syntax

/.../
A regex is enclosed between two forward slashes.

\
Cancels the special effect of a special character.

|
Alternative (or).

^
Start of line.

$
End of line.

[ ab ]
Match one of the characters inside the square brackets (a or b).

[ a-z ]
Match on a range of characters.

[^a]
Match if the character is not the one that is after the ^.

(myWord)
Match the group inside the parenthesis.

.
Match any character.

?
The preceding element is present 0 or 1 times.

{ 2 }
The preceding element is repeated a defined a number of times.

{ 0,3 }
The preceding element is repeated between 0 and 3 times.

*
The preceding element is repeated between 0 and infinite number of times.

+
The preceding element is repeated between 1 and infinite number of times.

Python

import re

MY_REGEX = ".*"

# Cache the regex (usefull if expression is reused multilple times)
re_compiled = re.compile(MY_REGEX)

# Check if a string matches the expression
if re_compiled.match(myString)
	print(f"Expression matched for: {myString}")

Useful Regex

IP

IP_REGEX = r"^([0-9]{1,3}\.){3}[0-9]{1,3}$"

ip a show eth0 | grep -oP '(?<=inet\s)\d+(.\d+){3}'
Extract IP from ip a.