Tag: regex
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Most referenced — #regex
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crawler 482
crawler_json 17
pre-tracking 6
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Regex Branch Reset Groups
A PCRE-specific construct (?|...) that resets capture group numbers across each alternative, so all branches share the same group indices.
1w ago
regex advanced
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
ereg() / eregi() — POSIX Regex Removed in PHP 7 PHP 3.0+
ereg() and eregi() were POSIX regex functions removed in PHP 7.0 — replace with preg_match() (PCRE) which is faster, more powerful, and the standard since PHP 4.
2mo ago
php beginner
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
Regex with grep, sed & awk
Three essential Linux text processing tools — grep for filtering lines, sed for stream editing, awk for field-based processing — all using regular expressions.
2mo ago
linux intermediate
Regular Expressions in JavaScript ES2015
JS regex syntax, flags (g, i, m, s, u, d), methods (test, match, matchAll, replace, split), and the critical differences from PHP's PCRE.
2mo ago
javascript 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
preg_match() / preg_replace() PHP 5.0+
PHP's PCRE regex functions — powerful but prone to ReDoS if patterns are not carefully constructed.
2mo ago
php intermediate
ReDoS (Regex Denial of Service)
A crafted input causes a regex with catastrophic backtracking to consume excessive CPU, making the application unresponsive.
CWE-1333 OWASP A5:2021
2mo ago
security intermediate
7.5
Regex in Loop PHP 5.0+
Compiling and executing the same regular expression on every iteration of a loop — hoist the pattern outside.
2mo ago
performance intermediate
Regex Performance Regex performance pitfalls — catastrophic backtracking (ReDoS), unnecessary captures, and poorly anchored patterns that scan more input than needed.
2mo ago
regex advanced