Dot isn’t everything
. skips line breaks unless you add /s, and even then it matches code points—not whole emojis. A single emoji can be two code units wide.
Grapheme gotcha
Regex Tester & Builder (JavaScript)
Pattern, Flags & Text
Tip: Ctrl/Cmd + K focuses the pattern. Ctrl/Cmd + Enter tests.
Results
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Matches & Groups
Shows match index, ranges, and capture groups (group ranges require d if supported).
Note: This uses your browser’s ECMAScript regex engine. Some PCRE tokens (e.g., atomic groups) aren’t supported.
Understanding Regular Expressions
A Regular Expression (Regex) is a pattern for matching text. Use them to search, validate, and replace.
Common examples
- Email:
^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ - IPv4:
^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$ - ISO date (YYYY-MM-DD):
^\d{4}-\d{2}-\d{2}$
Character classes
. any char · \w/\d/\s word/digit/space · [a-z] ranges
Anchors
^ start · $ end · \b word boundary
Groups & lookaround
(…) capture · (?:…) non-capture · (?=…)/(?!…) lookahead
Quantifiers
*/+/? · {m,n} · +? lazy
5 Fun Facts about Regex
Backtracking blowups
Patterns like (a+)+b on a thousand as can lock the engine, trying millions of paths. Small tweaks (a+b or lazy quantifiers) avoid it.
Performance trap
Indices flag = maps
The modern d flag returns start/end offsets for each capture. Great for highlighting matches without re-running the pattern.
Pinpoint groups
Order really matters
Alternations run left to right. In /(cat|catalog)/, “cat” wins first, so “catalog” never triggers the second branch.
Greedy choices
Born in neuroscience
Regex theory came from neurophysiologist Stephen Kleene in the 1950s. Decades later, grep (1973) brought it to Unix terminals.
Pattern history