层中的语法构造-允许递归吗? [英] Grammar construction in ply - recursion allowed?

问题描述

也许以前有人问过这个问题，但是我真的不知道要搜索什么.假设我想使用一些字符串来构建解析器.

This has maybe been asked before but I do not know what to search for, really. Suppose I have some string I'd like to build a parser with.

我有像a OR b，b OR C以及a OR (b AND c)这样的字符串.现在，嵌套的括号对我造成了麻烦，我不知道如何构造适当的p_*函数.允许递归吗?如果是这样，怎么办?

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到目前为止，这是我所拥有的:

I have strings like a OR b, b OR C but also a OR (b AND c). Now the nested parentheses cause trouble for me and I don't know how to construct the appropriate p_* functions. Is recursion allowed? If so, how?

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This is what I have thus far:

import ply.lex as lex
import ply.yacc as yacc

# List of token names.   This is always required
tokens = (
    'VARIABLE',
    'OR',
    'AND',
    'PAR_OPEN',
    'PAR_CLOSE',
)

# Regular expression rules for simple tokens
t_ignore        = ' \t'
t_VARIABLE      = r'\b[a-z]+\b'
t_OR            = r'\bOR\b'
t_AND           = r'\bAND\b'
t_PAR_OPEN      = r'\('
t_PAR_CLOSE     = r'\)'

def t_error(t):
    print("Illegal character '%s'" % t.value[0])
    t.lexer.skip(1)

# Build the lexer
lexer = lex.lex()

def p_term(p):
    '''term : VARIABLE OR VARIABLE
            | VARIABLE AND VARIABLE
            | PAR_OPEN VARIABLE AND VARIABLE PAR_CLOSE'''

    if p[2] == 'AND':
        p[0] = "".join([p[1], p[3]])

    for idx, val in enumerate(p):
        print(idx, val)

def p_error(p):
    print("Syntax error in input!")
    print(p)


parser = yacc.yacc()
res = parser.parse("(a AND b)")
print(res)

我也想用例如res = parser.parse("a OR (b AND c)")，但无济于事.

---

PS:这是基于 另一个人的问题 ，真的.

I'd also like to call it with e.g. res = parser.parse("a OR (b AND c)") but to no avail.

---

P.S.: This is based on another one's question, really.

推荐答案

表达式中的括号很常见.首先，请参考 PLY文档的第5部分，该示例提供了一个示例表达式解析的过程.是的，递归就是答案.

Parentheses in an expression is quite common. I will first refer you to part 5 of the PLY documentation which provides an example of nested expression parsing. Yes, recursion is the answer.

有几个短语用来表示表达式的最小元素".您可能会看到"atom"或"term"("terminal"的缩写)或"primary-expression".

There are several phrases that are used to mean the "smallest element of an expression." You might see "atom" or "term" (short for 'terminal') or "primary-expression".

在处理带括号的子表达式时，通常采用这种方法.编写一个语法规则，统一各种低级事物(例如，文字数字和变量名称)，然后在该点添加子表达式.

When dealing with parenthetical sub-expressions, this is generally the approach to take. Write a grammar rule that unifies the various low-level things (for example, literal numbers and variable names) and add the sub-expression at that point.

在此示例中，从PLY文档中，expression是最高级别的内容，并支持加法和减法.下一级是term，它支持乘法和除法.最低级别的是factor，它不支持任何操作，但可以统一NUMBER和带括号的子表达式.一个因素可能是7，但也可能是(7 + 2 * 3).

In this example, from the PLY docs, expression is the highest-level thing, and supports addition and subtraction. The next level down is term which supports multiplication and division. The lowest-level thing is the factor, which doesn't support any operations, but does unify NUMBER and parenthetical sub-expressions. A factor could be 7 but it could also be (7 + 2 * 3).

 expression : expression + term
            | expression - term
            | term

 term       : term * factor
            | term / factor
            | factor

 factor     : NUMBER
            | ( expression )

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