Regular Expression Grammar

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