Regex

What is Regex?

It’s a special language used to find, describe, and extract patterns in text. Instead of searching for a specific word, you search for a type of data.

The Anchors

The “Secret Guards” that tell you where to look.

Symbol	Secret Job	Example	Matches
^	The Front Guard: Must be at the very start.	^Hello	”Hello there”
$	The Back Guard: Must be at the very end.	end$	”The end”
\b	The Boundary: Must be a whole word.	\bcat\b	Finds “cat” but NOT “catnip”

Wildcards

The “Shape-shifters” that can be anything.

Symbol	Secret Job	Example	Matches
.	The Secret Agent: Any single character (except new line).	c.t	cat, c!t, c9t
\d	The Number Finder: Any single digit (0-9).	\d\d	42, 07, 99
\D	The Non-Number: Anything that ISN’T a digit.	\D	a, !, G
\w	The Word Character: Any letter, number, or underscore.	\w	a, Z, 5, _
\s	The Ghost: Any space, tab, or invisible character.	\d\s\d	5 5

Character Sets

The “Picky Eater” rules using square brackets [].

Symbol	Secret Job	Example	Matches
[abc]	The Menu: Must be one of these three.	[cp]at	cat, pat (NOT bat)
[a-z]	The Range: Any letter from a to z.	[a-z]!	a!, b!, z!
[^abc]	The Opposite: Anything NOT in this list.	[^0-9]	Any non-number

Quantifiers

The “Multipliers” that tell you how many times a symbol repeats.

Symbol	Secret Job	Example	Matches
*	The More-The-Merrier: 0 or more times.	lo*l	ll, lol, loool
+	The At-Least-One: 1 or more times.	lo+l	lol, loool (NOT ll)
?	The Optional: 0 or 1 time (maybe it’s there?).	colou?r	color, colour
{n}	The Precise: Exactly $n$ times.	\d{3}	123, 999
{min,max}	The Range: Between $min$ and $max$ times.	a{2,4}	aa, aaa, aaaa

Special Logic

The “Brain” of the pattern.

Symbol	Secret Job	Example	Matches
|	The “OR”: Matches either this or that.	cat|dog	cat, dog
( )	The Group: Bundles things together.	(ha)+	haha, hahaha
\	The Shield (Escape): Makes a magic symbol a normal letter.	\.	Just a period .

Dual-Agent: ^

This symbol changes jobs based on where it sits:

1. Outside Brackets (^Hello): I am the Start-of-Line guard.
2. Inside Brackets ([^abc]): I am the “NOT” symbol (Opposite Day).

Examples

import re
 
text = "Call me at 555-1234"
pattern = r"\d{3}-\d{4}"
 
# Find first
match = re.search(pattern, text) 
# Find all
all_matches = re.findall(pattern, text)
# Clean/Replace
clean_text = re.sub(r"[^a-zA-Z]", "", "Hello! 123") # "Hello"

// cargo.toml: regex = "1"
use regex::Regex;
 
let re = Regex::new(r"\d+").unwrap();
let text = "I have 10 apples";
 
let is_match = re.is_match(text); // true
let found = re.find(text).unwrap(); // "10"

Whenever you write a regex, use Regex101.com.

1. Paste your text.
2. Type your pattern.
3. It will explain exactly what is happening in the sidebar!