Tutorial voor reguliere expressies: van nul tot held

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.

Wat zijn reguliere expressies?

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

De basis: letterlijke karakters

The simplest regex is just literal text:

                Pattern: hello

                Matches: "hello" in "hello world"

                Does not match: "Hello" (case-sensitive by default)

Karakter klassen

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

Korte karakterklassen

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

Kwantificatoren: hoeveel?

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"

✅ Hebzuchtig versus lui

Quantifiers are "greedy" by default—they match as much as possible. Add ? to make them "lazy" (match as little as possible). .* vs .*?

Ankers: waar te matchen

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"

Groepen en vastleggen

Parentheses create groups for:

Applying quantifiers to multiple characters: (ab)+
Capturing matched text for later use
Creating alternatives: (cat|dog)

Voorbeeld: groepen vastleggen

                    Pattern: (\d{3})-(\d{3})-(\d{4})

                    Input: "555-123-4567"

                    Group 0 (full match): "555-123-4567"

                    Group 1: "555"

                    Group 2: "123"

                    Group 3: "4567"

Niet-vastleggende groepen

Use (?:...) when you need grouping but don't need to capture:

(?:https?|ftp):// // Groups but doesn't capture

Praktische patronen

E-mailvalidatie (basis)

^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

Matches: [email protected], [email protected]

Telefoonnummer (VS)

^$?(\d{3})$?[-.\s]?(\d{3})[-.\s]?(\d{4})$

Matches: (555) 123-4567, 555-123-4567, 555.123.4567

Wachtwoord (8+ tekens, hoofdletters, kleine letters, cijfers)

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$

Uses lookaheads to require different character types

URL

https?://[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}(/\S*)?

Matches: http://example.com, https://sub.domain.com/path

Vooruit en achterom kijken

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 ...

                // Match "foo" only if followed by "bar"

                foo(?=bar) // matches "foo" in "foobar", not in "foobaz"

                // Match $ amount (digit preceded by $)

                (?<=\$)\d+ // matches "100" in "$100"

Vlaggen/modificaties

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

Veelvoorkomende fouten

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.

🔧 Test uw patronen

Use our free RegEx Tester to experiment with patterns and see matches in real-time.

Open RegEx-tester →

Wanneer mag u Regex NIET gebruiken?

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() or str.startsWith() are clearer than regex.

Conclusie

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.

🔍 Tutorial voor reguliere expressies: van nul tot held