Tutorial de expresiones regulares: de cero a héroe

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.

¿Qué son las expresiones regulares?

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

Conceptos básicos: caracteres literales

The simplest regex is just literal text:

                Pattern: hello

                Matches: "hello" in "hello world"

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

Clases de personajes

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

Clases de caracteres taquigráficos

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

Cuantificadores: ¿cuántos?

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"

✅ Codicioso vs Perezoso

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

Anclas: dónde combinar

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"

Grupos y captura

Parentheses create groups for:

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

Ejemplo: captura de grupos

                    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"

Grupos sin captura

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

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

Patrones prácticos

Validación de correo electrónico (básico)

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

Matches: [email protected], [email protected]

Número de teléfono (EE. UU.)

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

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

Contraseña (8+ caracteres, mayúsculas, minúsculas, número)

^(?=.*[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

Mirar hacia adelante y hacia atrás

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"

Banderas/Modificadores

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

Errores comunes

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.

🔧 Prueba tus patrones

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

Abra el probador RegEx →

Cuándo NO usar expresiones regulares

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.

Conclusión

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 de expresiones regulares: de cero a héroe