[英]Context-free-grammar to represent regular expressions
I'm trying to make a context-free-grammar to represent simple regular expressions. 我正在尝试使上下文无关的语法来表示简单的正则表达式。 The symbols that I want is [0-9][az][AZ], and operators is "|", "()" and "."
我想要的符号是[0-9] [az] [AZ],运算符是“ |”,“()”和“”。 for concatenation, and for sequences for now I only want "*" later I will add "+","?", etc. I tried this grammar in javacc:
对于串联,对于现在的序列,我只想要“ *”,以后我将添加“ +”,“?”等。我在javacc中尝试了此语法:
void RE(): {}
{
FINAL(0) ( "." FINAL(0) | "|" FINAL(0))*
}
void FINAL(int sign): { Token t; }
{
t = <SYMBOL> {
if ( sign == 1 )
jjtThis.val = t.image + "*";
else
jjtThis.val = t.image;
}
| FINAL(1) "*"
| "(" RE() ")"
}
The problem is in FINAL function the line | FINAL(1) "*"
问题是最终功能线
| FINAL(1) "*"
| FINAL(1) "*"
that gives me a error Left recursion detected: "FINAL... --> FINAL...
. Putting "*" on the left of FINAL(1) resolve the problem but this is not what I want.. | FINAL(1) "*"
给我一个错误Left recursion detected: "FINAL... --> FINAL...
在FINAL(1)左侧放置” *“可以解决问题,但这不是我要解决的问题想..
I already tried to read the article from wikipedia to remove left recursion but I really don't know how to do it, can someone help? 我已经尝试过阅读Wikipedia上的文章,以消除左递归,但是我真的不知道该怎么做,有人可以帮忙吗? :s
:s
The following takes care of the left recursion 以下照顾左递归
RE --> FACTOR ("." FINAL | "|" FINAL)*
FINAL --> PRIMARY ( "*" )*
PRIMARY --> <SYMBOL> | "(" RE ")"
However, that won't give . 但是,那不会给。 precedence over |
优先于| .
。 For that you can do the following
为此,您可以执行以下操作
RE --> TERM ("|" TERM)*
TERM --> FINAL ("." FINAL)*
FINAL --> PRIMARY ( "*" )*
PRIMARY --> <SYMBOL> | "(" RE ")"
The general rule is 一般规则是
A --> A b | c | d | ...
can be transformed to 可以转化为
A --> B b*
B --> c | d | ...
where B is a new nonnterminal. 其中B是一个新的非终结符。
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