Aprende Expresiones Regulares

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Think of it as a smart "find" tool — instead of searching for exact text, you search for patterns.

Regex is used everywhere in programming: - Validating input (emails, phone numbers, passwords) - Searching text (find all URLs in a document) - Extracting data (pull dates from a log file) - Find & replace (reformat text programmatically)

Character classes let you match any one of a set of characters.

• ●\d — any digit (0-9)
• ●\w — any "word" character (letters, digits, underscore)
• ●\s — any whitespace (spaces, tabs, newlines)
• ●[abc] — any one of: a, b, or c
• ●[a-z] — any lowercase letter
• ●[^abc] — any character except a, b, or c
• ●. (dot) — any character except newline

Uppercase versions are the opposite: \D = non-digit, \W = non-word, \S = non-whitespace.

Quantifiers specify how many of the preceding element to match.

By default, quantifiers are greedy (match as much as possible). Add ? to make them lazy (match as little as possible).

Anchors don't match characters — they match positions.

• ●^ — start of string (or line, with m flag)
• ●$ — end of string (or line, with m flag)
• ●\b — word boundary (between \w and \W)
• ●\B — non-word boundary

These are essential for ensuring your regex matches the right thing, not just any occurrence.

Parentheses create groups that can capture matched text.

• ●(abc) — capturing group (stores the match)
• ●(?:abc) — non-capturing group (groups without storing)
• ●(?abc) — named group (access by name)
• ●\1 — backreference (match the same text as group 1)

Groups are also used with quantifiers: (ha)+ matches "ha", "haha", "hahaha".

The pipe | acts as an OR operator.

• ●cat|dog — matches "cat" or "dog"
• ●(red|blue|green) car — matches "red car", "blue car", or "green car"

Without parentheses, alternation applies to the entire pattern on each side. Use groups to limit the scope.

Lookaround assertions check what's around a match without including it.

• ●(?=abc) — positive lookahead: followed by "abc"
• ●(?!abc) — negative lookahead: NOT followed by "abc"
• ●(?<=abc) — positive lookbehind: preceded by "abc"
• ●(? — negative lookbehind: NOT preceded by "abc"

These are powerful for extracting text in specific contexts.

Flags modify how the regex engine processes the pattern.