Regex terms
Powerful pattern matching — with great power comes great confusion
Regular expressions are a compact language for describing patterns in text — extraordinarily powerful, notoriously hard to read, and present in virtually every programming language. This category covers syntax, anchors, groups, lookaheads, quantifiers, and the performance traps like catastrophic backtracking that turn a useful pattern into a production incident.
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Anchors & Word Boundaries PHP 5.3+
Zero-width assertions that match positions rather than characters — ^ matches start of string, $ matches end, \b matches a word boundary — ensuring patterns match in the correct location.
2mo ago
regex beginner
Capture Groups & Backreferences PHP 5.3+
Parentheses in a regex pattern create capture groups that store matched substrings for extraction or reuse — backreferences let you match the same text again later in the pattern or replacement string.
2mo ago
regex beginner
Catastrophic Backtracking PHP 5.3+
A regex engine failure mode where certain patterns combined with specific inputs cause exponential backtracking, making the match take seconds or minutes — a common denial-of-service vector called ReDoS.
2mo ago
regex intermediate
Greedy vs Lazy Quantifiers PHP 5.3+
Greedy quantifiers (*, +, ?) match as much as possible; lazy quantifiers (*?, +?, ??) match as little as possible — the difference determines which text is captured when multiple matches are possible.
2mo ago
regex beginner
Lookahead & Lookbehind PHP 5.3+
Zero-width assertions that match a position based on what precedes or follows it, without consuming characters — allowing conditions on surrounding context without including that context in the match.
2mo ago
regex intermediate
Regex Flags (i, g, m, s, x, u) PHP 5.3+
Modifiers appended to a regex pattern that change matching behaviour — case-insensitivity, multiline mode, dotall mode, extended whitespace, and Unicode awareness.
2mo ago
regex beginner
Named Capture Groups PHP 5.3+
(?P<n>pattern) in PHP/PCRE for self-documenting regex — accessing matches by name instead of positional index.
2mo ago
regex intermediate
PCRE in PHP PHP 5.0+
preg_match, preg_match_all, preg_replace, preg_split — and checking === false to distinguish errors from no-match.
2mo ago
regex intermediate
Regex Flags & Modifiers PHP 5.3+
PCRE modifiers in PHP — i (case-insensitive), m (multiline), s (dotall), x (verbose), u (Unicode) — and their inline equivalents (?imsx).
2mo ago
regex intermediate
Unicode & Multibyte Regex PHP 5.3+
The u flag enables UTF-8 mode and \p{} Unicode property classes — essential for correctly matching text in any language.
2mo ago
regex intermediate
Common Regex Patterns PHP 5.3+
Practical regex patterns for email, URL, phone, IP address, dates, and identifiers — with caveats about when to use dedicated validators instead.
2mo ago
regex beginner
Lookahead & Lookbehind
Zero-width assertions that match a position based on what precedes or follows — without consuming characters, enabling context-sensitive matching.
2mo ago
regex advanced
Regex Performance Regex performance pitfalls — catastrophic backtracking (ReDoS), unnecessary captures, and poorly anchored patterns that scan more input than needed.
2mo ago
regex advanced
Regex Syntax PHP 5.3+
Regular expression syntax — anchors, quantifiers, character classes, groups, and alternation — the building blocks for pattern matching in PHP and all languages.
2mo ago
regex intermediate