Regular expressions (regex) are one of the most powerful tools in a developer's arsenal—and one of the most feared. This tutorial breaks down regex into digestible pieces, building from basic concepts to patterns you can use in real projects.
🎯 By the end of this tutorial, you'll be able to: Write patterns for email validation, password requirements, phone numbers, and more. You'll understand quantifiers, groups, lookaheads, and know when to use (or avoid) regex.
Que sont les expressions régulières ?
Regular expressions are patterns that describe sets of strings. They're used to:
- Search: Find text matching a pattern
- Validate: Check if input matches expected format
- Extract: Pull specific parts from text
- Replace: Transform text based on patterns
Les bases : les personnages littéraux
The simplest regex is just literal text:
Matches: "hello" in "hello world"
Does not match: "Hello" (case-sensitive by default)
Classes de personnages
Match any single character from a set:
| Pattern | Matches | Example |
|---|---|---|
[abc] |
a, b, or c | "cat" matches c |
[a-z] |
Any lowercase letter | "Hello" matches e, l, l, o |
[A-Z] |
Any uppercase letter | "Hello" matches H |
[0-9] |
Any digit | "abc123" matches 1, 2, 3 |
[^abc] |
NOT a, b, or c | "dog" matches d, o, g |
Classes de caractères sténographiques
| Shorthand | Equivalent | Description |
|---|---|---|
\d |
[0-9] |
Any digit |
\D |
[^0-9] |
Not a digit |
\w |
[a-zA-Z0-9_] |
Word character |
\W |
[^a-zA-Z0-9_] |
Not a word character |
\s |
[ \t\n\r] |
Whitespace |
\S |
[^ \t\n\r] |
Not whitespace |
. |
(almost anything) | Any character except newline |
Quantificateurs : combien ?
| Quantifier | Meaning | Example |
|---|---|---|
* |
0 or more | ab*c matches "ac", "abc", "abbc" |
+ |
1 or more | ab+c matches "abc", "abbc", not "ac" |
? |
0 or 1 | colou?r matches "color" and "colour" |
{n} |
Exactly n | \d{4} matches "2026" |
{n,} |
n or more | \d{2,} matches "12", "123", "1234" |
{n,m} |
Between n and m | \d{2,4} matches "12", "123", "1234" |
✅ Gourmands ou paresseux
Quantifiers are "greedy" by default—they match as much as possible. Add ? to make them
"lazy" (match as little as possible). .* vs .*?
Ancres : où correspondre
| Anchor | Position | Example |
|---|---|---|
^ |
Start of string/line | ^Hello matches "Hello world", not "Say Hello" |
$ |
End of string/line | world$ matches "Hello world", not "world peace" |
\b |
Word boundary | \bcat\b matches "cat" but not "category" |
Groupes et capture
Parentheses create groups for:
- Applying quantifiers to multiple characters:
(ab)+ - Capturing matched text for later use
- Creating alternatives:
(cat|dog)
Exemple : Capturer des groupes
Input: "555-123-4567"
Group 0 (full match): "555-123-4567"
Group 1: "555"
Group 2: "123"
Group 3: "4567"
Groupes non capturants
Use (?:...) when you need grouping but don't need to capture:
Modèles pratiques
Validation des e-mails (de base)
Matches: [email protected], [email protected]
Numéro de téléphone (États-Unis)
Matches: (555) 123-4567, 555-123-4567, 555.123.4567
Mot de passe (8+ caractères, majuscules, minuscules, chiffres)
Uses lookaheads to require different character types
URL
Matches: http://example.com, https://sub.domain.com/path
Anticipation et analyse derrière
Match based on what comes before or after, without including it in the match:
| Type | Syntax | Description |
|---|---|---|
| Positive Lookahead | (?=...) |
Followed by ... |
| Negative Lookahead | (?!...) |
NOT followed by ... |
| Positive Lookbehind | (?<=...) |
Preceded by ... |
| Negative Lookbehind | (? |
NOT preceded by ... |
foo(?=bar) // matches "foo" in "foobar", not in "foobaz"
// Match $ amount (digit preceded by $)
(?<=\$)\d+ // matches "100" in "$100"
Indicateurs/Modificateurs
| Flag | Description |
|---|---|
i |
Case-insensitive matching |
g |
Global - find all matches, not just first |
m |
Multiline - ^ and $ match line boundaries |
s |
Dotall - . matches newlines too |
Erreurs courantes
- Forgetting to escape:
.,*,+,?, etc. have special meaning. Use\.to match a literal period. - Greedy matching:
.*matches too much. Use.*?for lazy matching. - Missing anchors:
\d{4}matches "2026" anywhere. Use^\d{4}$for exact match. - Overcomplicated patterns: Sometimes string methods or multiple simple patterns are clearer.
🔧 Testez vos modèles
Use our free RegEx Tester to experiment with patterns and see matches in real-time.
Ouvrir le testeur RegEx →Quand NE PAS utiliser Regex
- Parsing HTML/XML: Use a proper parser. Regex can't handle nested tags correctly.
- Complex validation: Email RFC is incredibly complex. Use a library.
- Simple tasks:
str.includes()orstr.startsWith()are clearer than regex.
Conclusion
Regular expressions are like a superpower—incredibly useful once you learn them, but easy to misuse. Start with simple patterns, test incrementally, and don't be afraid to use comments or break complex patterns into pieces.
The key to mastering regex is practice. Use the patterns in this tutorial as building blocks, experiment with variations, and soon you'll be writing patterns confidently.