Regex Cheat Sheet & Tester

Regular expressions ("regex") are tiny domain-specific languages for describing patterns in text. A pattern like \b\d{3}-\d{4}\b describes "three digits, a hyphen, four digits, as a whole word." Once written, regex engines built into PHP, JavaScript, Python, Java, Ruby, Go, Rust, and every modern command-line tool can find, validate, split, or replace text matching that pattern at high speed. For developers, regex is the difference between writing thirty lines of string-walking code and writing one line that does the same job, faster.

Where regex shines

The honest answer is: anywhere you'd otherwise write a parser-by-hand for a small, well-defined format. Validating that an email looks roughly right, extracting all URLs from a markdown document, splitting a phone number into country code and local digits, finding all occurrences of a UUID in a log file, replacing every Singapore NRIC with a redacted placeholder before sending logs offshore — these are all 1-to-3-line regex problems. The tool above ships with 80+ patterns covering most of the common cases so you don't have to redesign them from scratch each time.

Where regex stops being the right tool

The classic example is parsing HTML or JSON with regex. These languages are nested, and regex (with the textbook computer-science definition of "regular") cannot match arbitrarily nested structures. Real-world engines like PCRE add features that bend the rule — backreferences, recursive subpatterns — but the maintainability cost is enormous. If you find yourself writing more than 60 characters of regex for a single pattern, or nesting capture groups three levels deep, stop and use a proper parser. DOMDocument in PHP, BeautifulSoup in Python, cheerio in Node — they exist for a reason.

ASEAN-specific patterns

Many regex collections online focus on US/UK formats. The library here includes patterns calibrated for Southeast Asian use: Singapore NRIC (S/T/F/G/M-prefix plus seven digits plus a checksum letter), Singapore postal code (six digits, but only certain three-digit prefixes are valid sector codes), Malaysia IC number (twelve digits in YYMMDD-PB-#### format), Indonesian KTP (sixteen digits), Singapore phone numbers (eight digits starting with 6, 8, or 9), and Singaporean GST registration numbers. Bookmark these — the regional-format ones are the patterns most often missing from the generic Stack Overflow answers.

Flags, anchors, and the global flag

Two small details cause more confusion than they should. First, the g (global) flag does not change what a pattern matches — it only tells the engine "find all matches" rather than "find the first." If your live tester is showing only one highlight when you expect many, you have probably forgotten the g. Second, anchors. ^ and $ mean "start" and "end" of the input by default. Add the m (multi-line) flag and they become "start" and "end" of each line. Knowing which one you need before you start writing the pattern saves hours of "why isn't this matching" debugging.

When regex is the wrong tool

Three flags that say "stop and reach for a real parser": you are matching balanced brackets at arbitrary depth, you are extracting structured data from JSON or HTML, or your pattern has grown past one screen of text. Use a proper parser library for those. Regex is brilliant for flat, regular patterns inside a single line; it is fragile and slow for grammars.