What are some of the practical examples that require back-tracking (for whatever definition of "require")? The example I was once being told about was parsing a

What are some of the practical examples that require back-tracking (for whatever definition of "require")? The example I was once being told about was parsing assignment expressions: Expr ::= CanBeLhsOfAssignExpr | CanBeLhsOfAssignExpr "=" Expr | CanNotBeLhsOfAssignExpr but in practice, it's easily parsed in the following manner: def parse_Expr(self): # parse_ExprInner parses CanBeLhsOfAssignExpr or CanNotBeLhsOfAssignExpr cases result = self.parse_ExprInner() if self.peekToken() == TOKEN_ASSIGN: if result.is_CanBeLhsOfAssignExpr: token = self.skipToken() # assignment is right-associative rhs = self.parse_Expr() result = self.make_AssignExpr(token, result, rhs) else: self.raiseParseError("{} can't be target of assignment", result) return result The idea is to parse the prefix under "relaxed" rules, delaying the well-formedness check until it's actually needed.