在不回溯的情况下解析 C 赋值表达式的诀窍是什么？

问：

自上而下解析 C 必须在 a 和 a 之间进行选择。（不幸的是，很复杂）BNF 是：assignment-expressionconditional-expressionunary-expression

<assignment-expression>     ::= <conditional-expression>
                              | <unary-expression> <assignment-operator> <assignment-expression>
<conditional-expression>    ::= <logical-or-expression> 
                              | <logical-or-expression> ? <expression> : <conditional-expression>
<logical-or-expression>     ::= <logical-and-expression> 
                              | <logical-or-expression> || <logical-and-expression>
<logical-and-expression>    ::= <inclusive-or-expression> 
                              | <logical-and-expression> && <inclusive-or-expression>
<inclusive-or-expression>   ::= <exclusive-or-expression> 
                              | <inclusive-or-expression> '|' <exclusive-or-expression>
<exclusive-or-expression>   ::= <and-expression> 
                              | <exclusive-or-expression> ^ <and-expression>
<and-expression>            ::= <equality-expression> 
                              | <and-expression> & <equality-expression>
<equality-expression>       ::= <relational-expression> 
                              | <equality-expression> == <relational-expression> 
                              | <equality-expression> != <relational-expression>
<relational-expression>     ::= <shift-expression>
                              | <relational-expression> < <shift-expression>
                              | <relational-expression> > <shift-expression>
                              | <relational-expression> <= <shift-expression>
                              | <relational-expression> >= <shift-expression>
<shift-expression>          ::= <additive-expression>
                              | <shift-expression> << <additive-expression>
                              | <shift-expression> >> <additive-expression>
<additive-expression>       ::= <multiplicative-expression>
                              | <additive-expression> + <multiplicative-expression>
                              | <additive-expression> - <multiplicative-expression>
<multiplicative-expression> ::= <cast-expression>
                              | <multiplicative-expression> * <cast-expression>
                              | <multiplicative-expression> / <cast-expression>
                              | <multiplicative-expression> % <cast-expression>
<cast-expression>           ::= <unary-expression>
                              | ( <type-name> ) <cast-expression>
<unary-expression>          ::= <postfix-expression>
                              | ++ <unary-expression>
                              | -- <unary-expression>
                              | <unary-operator> <cast-expression>
                              | sizeof <unary-expression>
                              | sizeof <type-name>
<postfix-expression>        ::= <primary-expression>
                              | <postfix-expression> [ <expression> ]
                              | <postfix-expression> ( {<assignment-expression>}* )
                              | <postfix-expression> . <identifier>
                              | <postfix-expression> -> <identifier>
                              | <postfix-expression> ++
                              | <postfix-expression> --
<primary-expression>        ::= <identifier>
                              | <constant>
                              | <string>
                              | ( <expression> )

请注意，a 满足该规则。unary-expressioncast-expression

如果输入是，并且解析器首先尝试分支，则输入只需停在 即可满足规则。解析器必须回溯并考虑分支才能发现该部分。X=3conditional-expressionprimary-expressionXunary-expression assignment-operator assignment-expression=3

如果输入是，并且解析器首先尝试分支，则永远不会找到，并且解析器必须回溯并考虑分支以发现规则。X==Yunary-expressionassignment-operatorconditional-expressionequality-expression

我不明白如何在任何地方左分解 BNF 以避免回溯。递归和回溯是不可避免的吗？还是有诀窍？

编辑：为了回应要求简化语法的评论，下面将演示 vs。 问题，但只是为了证明简化的语法如何需要回溯，对于那些不想摸索完整语法的人来说。为避免在这种简化的语法中回溯而进行的任何修复都可能无法使用完整语法。X=3X==Y

<assignment-expression>     ::= <conditional-expression>
                              | <unary-expression> <assignment-operator> <unary-expression>
<conditional-expression>    ::= <unary-expression> == <unary-expression>
<unary-expression>          ::= <identifier>
<assignment_operator>       ::= '='

如果输入是，并且解析器首先查找，则找不到，因此解析器必须回溯，而输入则无需回溯即可工作。X=3conditional-expression==X==Y

如果输入是，并且解析首先查找，则输入是可以接受的，而输入将需要回溯才能匹配。X=3unary-expressionX==Yconditional-expression