CODEWITHSUNDEEP
javascript
DEN
DEN

Reputation: 1893

How to match a input parenthesis with regular expression in JavaScript?

I have no idea on matching the input parenthesis with JavaScript.

Input string example:

(pen)
((pen) and orange)

it should return false if the input string is like the following:

(pen
pen)
(pen) and orange)
((pen and orange )
((pen) and orange 
)(pen and orange )(
(pen and )orange()

Upvotes: 0

Views: 1613

Answers (4)

Chad
Chad

Reputation: 9859

I made a node library call balanced to make this a bit more sane, if you wanted to just get balanced outer matches you can do this

balanced.matches({source: source, open: '(', close: ')'})

my use case was a bit more complicated, and required me to do replacements and support comments.

Upvotes: 0

Chris Morgan
Chris Morgan

Reputation: 90752

Regular expressions would be messy. It's much easier to go through with a simple counter.

function parenthesesBalanced(string) {
    var count = 0;

    for (var i = 0, l = string.length; i < l; i++) {
        var char = string.charAt(i);

        if (char == "(") {
            // Opening parenthesis is always OK
            count++;
        } else if (char == ")") {
            // If we're at the outer level already it's not valid
            if (count == 0) return false;
            count--;
        }
    }

    return (count == 0);
}

Upvotes: 3

user187291
user187291

Reputation: 53940

replace every group of "left paren - some chars - right paren" with nothing, until there is no more groups. If the resulting string contains a parenthesis, the parens were not balanced. 

balancedParens = function(str) {
    var q;   
    do {
      q = str;
      str = str.replace(/\([^()]*\)/g, '');
    } while(q != str);
    return !str.match(/[()]/);
}

    a = "foo ((and) bar and (baz) quux) and (blah)";
    b = "(dddddd()";

alert(balancedParens(a))
alert(balancedParens(b))

http://jsfiddle.net/gvGGT/

It's not possible to match a balanced string with a single regexp in javascript, because JS dialect doesn't support recursive expressions.

Upvotes: 1

Zack Bloom
Zack Bloom

Reputation: 8417

It is a known hard problem to match parens with regular expressions. While it is possible, it's not particularly efficient.

It's much faster simply to iterate through the string, maintaining a counter, incrementing it every time you hit an open paren, and decrementing it every time you hit a close paren. If the counter ever goes below zero, or the counter is not zero at the end of the string, it fails.

Upvotes: 1

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