[英]Unambiguous Grammar for Regular Expressions
I'm trying to develop a recursive decent parser for regular expressions for a homework assignment.我正在尝试为家庭作业的正则表达式开发一个递归体面的解析器。 I just wanted to ask the community if the grammar I've developed is correct or if I'm on the right track:
我只是想问问社区我开发的语法是否正确,或者我是否走在正确的轨道上:
-= Regex Grammar (EBNF) =-
<start> -> <expr> '\n'
<expr> -> <expr> { '|' <term> } // Union
| <expr> { <expr> } // Concatenation
| <expr> '*' // Closure
| <term>
<term> -> '(' <expr> ')' | <char> // Grouping
| <char>
<char> -> a|b|c| ... |z
A few guidelines:一些指导方针:
1. Precedence: In the order listed (highest to lowest) Closure, Concatenation, Union 1. 优先级:按照列出的顺序(从高到低)闭包、串联、并集
2. Associativity: Closure is right-associative; 2. 结合性:闭包是右结合的; Concatenation/Union are left-associative
连接/联合是左关联的
3. Must support grouping with parens 3.必须支持带括号的分组
My Question: Does the grammar (above) meet the guidelines?我的问题:语法(以上)是否符合准则? I feel certain but I'm not 100% and was hoping a few seasoned eyes could point out some issues/errors.
我有把握,但我不是 100%,希望一些经验丰富的眼睛能指出一些问题/错误。
TIA Noob TIA 菜鸟
<start>
<expr>
<expr><expr>
<expr><expr><expr>
<term><term><term>
'abc'
This is ambiguous, because in the third step you can either expand the first <expr>
or the latter one.这是模棱两可的,因为在第三步中,您可以展开第一个
<expr>
或后一个。 You should be able to work around that by removing您应该可以通过删除来解决这个问题
<expr> -> <expr> { <expr> }
and create并创建
<term> -> <term> <expr>
instead.反而。
You are repeating yourself here你在这里重复自己
<term> -> '(' <expr> ')' | <char> // Grouping
| <char>
(you have <char>
two times, did you mean to have it '(' <expr> ')' '|' <char>
in the first rule?) I think it would be clearer to remove (你有两次
<char>
,你的意思是在第一条规则中有它'(' <expr> ')' '|' <char>
吗?)我认为删除它会更清楚
<term> -> '(' <expr> ')'
and create并创建
<expr> -> '(' <expr> ')'
instead.反而。
Then you also need to add quotation marks around the characters in <char>
.然后您还需要在
<char>
中的字符周围添加引号。
This is what I see from quickly looking through your EBNF, it's been a while since I was studying this myself so some of my corrections might be wrong.这是我快速浏览您的 EBNF 所看到的,我已经有一段时间没有自己研究这个了,所以我的一些更正可能是错误的。
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