RegexEditor.dev Logo

Regex Editor & Tester

Build, test, and debug regular expressions

← Back to BlogFormal RE›Concatenation
← prevnext →
8 / 44
aεε is the concatenation identity: aε = aConcatenationFormal RE✨ Emoji Combos

Concatenating any RE with ε leaves it unchanged. aε ≡ εa ≡ a. This is one of the fundamental algebraic laws of regular expressions.

✓ Accepts
a
✗ Rejects
εaeaa
Loading editor…