CODEWITHSUNDEEP
pythonregex
Itzik984
Itzik984

Reputation: 16804

Unbalanced parenthesis python

I have the following code:

def commandType(self):
    import re
    print self.cmds[self.counter]
    if re.match("@",self.cmds[self.counter]):
        return Parser.A_COMMAND

    elif re.match('(',self.cmds[self.counter]):
        return Parser.L_COMMAND

    else:
        return Parser.C_COMMAND

and on this line: elif re.match('(',self.cmds[self.counter]):

I'm getting an error.

What am I doing wrong?

Upvotes: 2

Views: 46530

Answers (3)

Matt Ball
Matt Ball

Reputation: 359986

Parentheses have special meaning in regular expressions. You can escape the paren but you really do not need a regex at all for this problem:

def commandType(self):
    print self.cmds[self.counter]
    if '@' in self.cmds[self.counter]):
        return Parser.A_COMMAND

    elif '(' in self.cmds[self.counter]:
        return Parser.L_COMMAND

    else:
        return Parser.C_COMMAND

Upvotes: 9

Gareth McCaughan
Gareth McCaughan

Reputation: 19981

The language of regular expressions gives a special meaning to ( (it's used for starting a group). If you want to match a literal left-parenthesis, you need to escape it with a backslash: elif re.match(r'\(', ....

(Why r'...' rather than just '...'? Because in ordinary strings, backslashes are also used for escaping control characters and suchlike, and you need to write \\ to get a single backslash into the string. So you could instead write elif re.match('\\(', .... It's better to get into the habit of using r'...' strings for regular expressions -- it's less error-prone.)

Upvotes: 2

Zaur Nasibov
Zaur Nasibov

Reputation: 22679

The parenthesis '(' and ')' are used as grouping mechanism and scope operators in regexps. You have to escape them (and any other control symbols) via backslash, e.g. '\('.

Upvotes: 8

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