| regular‑expression | → | alternative |
| alternative | → | catenation (| catenation)* |
| catenation | → | repetition repetition* |
| repetition | → | primary (* | + | ?)? |
| primary | → | ( alternative ) | escape | ^ | - | [^\0\r\n{}\\()[]|*+?.^-] | . | class |
| class | → | [ (^? range*) ] |
| range | → | char (- char | <empty>) |
| char | → | ( | ) | [ | | | * | + | ? | . | escape | ^ | - | [^\0\r\n{}\\()[]|*+?.^-] |