Regex guides
Long-form guides covering regex fundamentals, performance, language-specific deep dives, and validation patterns. Read in any order.
Regex fundamentals
Learn regular expressions from scratch — character classes, quantifiers, anchors, groups, and your first pattern.
Anchors — ^ $ \b explained
How anchors work in regex: start of string, end of string, word boundary, and how multiline mode changes them.
Quantifiers — greedy vs lazy
How *, +, ?, {n}, and their lazy versions work, with practical examples.
Character classes
Square bracket classes, shorthand classes (\d \w \s), and Unicode property escapes.
Lookarounds — assertions explained
Positive and negative lookahead and lookbehind, with real examples.
Common regex mistakes
The most frequent regex pitfalls and how to avoid them.
Regex performance & ReDoS
Why some patterns are slow, what catastrophic backtracking is, and how to fix it.
Regex in Python
Complete guide to Python's re module: compile, match, search, findall, sub, named groups, verbose mode.
Regex in JavaScript
Modern JavaScript regex: RegExp, named groups, lookbehind, Unicode property escapes, sticky flag.
Regex flags reference
Every regex flag explained: g, i, m, s, u, x, y, d. With examples and language-specific notes.
Validation patterns
When to validate with regex, when to use a parser instead. Battle-tested patterns for the common fields.
Regex substitution
Search and replace with back-references, named groups, function replacements, across all the major engines.
Capture groups — every kind
Capturing, non-capturing, named, atomic — when to use each.
Validating Indian IDs
Aadhaar, PAN, GSTIN, UPI, IFSC — complete guide with checksum notes.
Converting flavors
Porting regex between Python, JavaScript, PCRE, Java, .NET, Go, Ruby.
Groups and capture groups
Capture, non-capture, named groups, and back-references.
Escaping special characters
Which characters need escaping and a reference table.
Advanced character classes
POSIX classes, Unicode categories, intersection.
Regex flags deep dive
Every flag: g, i, m, s, u, y, d, x.
Regex in Go
RE2 engine, limitations, and idioms.
Regex in Java
Pattern, Matcher, double-escaping pain.
Regex in C# / .NET
The most feature-rich engine.