Regex Editor & Tester

Build, test, and debug regular expressions

Editor AI Generator

About Help Contact Terms Privacy

← Back to BlogFormal RE›Primitives

← prev next →

3 / 44

aSingle literal characterPrimitivesFormal RE✨ Emoji Combos

A single symbol matches exactly that one-character string. Formal RE is always case-sensitive — there are no flags.

✓ Accepts

a

✗ Rejects

εbaaAa

Loading editor…

RegexTester.dev

Regex Debugger — step through every backtrack Regex Benchmark Suite — compare patterns Blog: Regex Debugger Blog: Benchmark Suite Blog: Automata Theory Flavor Pattern Gallery

Featured on

© 2026 RegexTester.dev — free online regex testerNo signup · No cookies · Runs in your browser

Regular Expression Pattern

Flavor

/

/

Test String

Annotation

Unable to parse pattern

Active Flags:

gglobal: Don't return after first match

mmultiline: ^ and $ match start/end of each line

Match Results

0 matches

No matches found

Quick Reference