CODEWITHSUNDEEP
python
rebel
rebel

Reputation: 55

Python nicest way to take boolean expression string apart

I have a boolean expression string, that I would like to take apart:

condition = "a and (b or (c and d))"

Or let's say:
I want to be able to access the string contents between two parenthesis.
I want following outcome:

"(b or (c and d))"
"(c and d)"

I've tried the following with regular expressions (not really working)

x = re.match(".*(\(.*\))", condition)
print x.group(1)

Question:
What is the nicest way to take a boolean expression string apart?

Upvotes: 4

Views: 203

Answers (4)

Bull
Bull

Reputation: 11941

If your requirements are fairly simple, you don't really need a parser. Matching parentheses can easily be achieved using a stack.

You could do something like the following:

condition = "a and (b or (c and d))"

stack = []

for c in condition:
    if c != ')':
        stack.append(c)
    else:
        d = c
        contents = []
        while d != '(':
            contents.insert(0, d)
            d = stack.pop()
        contents.insert(0, d)
        s = ''.join(contents)
        print(s)
        stack.append(s)

produces:

(c and d)
(b or (c and d))

Upvotes: 2

Adrian Panasiuk
Adrian Panasiuk

Reputation: 7343

Build a parser:

 Condition ::= Term Condition'
 Condition' ::= epsilon | OR Term Condition'
 Term  ::=  Factor Term'
 Term' ::= epsilon | AND Factor Term'
 Factor ::= [ NOT ] Primary
 Primary ::= Literal | '(' Condition ')'
 Literal ::= Id

Upvotes: 0

Cyrille
Cyrille

Reputation: 14553

Like everyone said, you need a parser.

If you don't want to install one, you can start from this simple top-down parser (take the last code sample here)

Remove everything not related to your need (+, -, *, /, is, lambda, if, else, ...). Just keep parenthesis, and, or. You will get a binary tree structure generated from your expression. The tokenizer use the build-in tokenize (import tokenize), which is a lexical scanner for Python source code but works just fine for simple cases like yours.

Upvotes: 2

Daniel Roseman
Daniel Roseman

Reputation: 599856

This is the sort of thing you can't do with a simple regex. You need to actually parse the text. pyparsing is apparently excellent for doing that.

Upvotes: 8

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