grammar.txt

regexpTopLevel ::= '^' regexp '$'
                 | '^' regexp
                 | regexp '$'
                 | regexp

regexp ::= regexpNoUnion '|' regexp
         | regexpNoUnion

regexpNoUnion ::= regexpNoConcat regexpNoUnion
                | regexpNoConcat
                | { empty }

regexpNoConcat ::= regexpBasic regexpOp

regexpOp ::= '*' regexpOp
           | '+' regexpOp
           | '?' regexpOp
           | { empty }

regexpBasic ::= '[' charClass ']'
              | singleChar
              | singleEscapedChar
              | '(' regexp ')'

charClass ::= '^' charRangesOrSingles
            | charRangesOrSingles

charRangesOrSingles ::= singleChar charRangeOrSingles
                      | singleEscapedChar charRangeOrSingles
                      | charRange charRangeOrSingles
                      | singleChar
                      | singleEscapedChar
                      | charRange

charRange ::= singleChar '-' singleChar

singleChar ::= a, b, c, ..., A, B, C, ... 0, 1, 2, ..., !, @, #, ..., '.'

singleEscapedChar ::= '\' { s, S, \, (, ), {, }, [, ], ^, ., -, {space}, n, t }