Use Regex to remove outer parentheses from nested expression, but leave inner parentheses?

Question

I'm working on figuring out a good regular expression that would take a value such as:

Transformer Winding Connections (Wye (Star) or Delta)

and would match:

Wye (Star) or Delta

What I have so far is:

      string longName = "Transformer Winding Connections (Wye (Star) or Delta)";

      // Match everything until first parentheses
      Regex nameRegex = new Regex(@"([^(]*)");

      Match nameMatch = nameRegex.Match(longName);

      // Match everything from first parentheses on    
      Regex valueRegex = new Regex(@"\(.+\)");

      Match valueMatch = valueRegex.Match(longName);

valueMatch is returning:

(Wye (Star) or Delta)

Is there some clever way to only remove the first set of parentheses in C#?

How about removing the *first* and *last* parantheses without regex? — EZI, Jul 21 '15 at 18:25
Does it have to be a regex? Otherwise couldn't you potentially get away with utilizing `longName.IndexOf("(")` and `longName.LastIndexOf(")")`? — Sven Grosen, Jul 21 '15 at 18:25
@bdimag there shouldn't be more than one or two levels, but it could have multiple parentheses on that level, if that makes sense. Like, it could be "Transformer Winding Connections (Wye (Star) or Delta (Something))". — Raymond, Jul 21 '15 at 18:30
@AvinashRaj your solution matches the outer parentheses as well, though. I'm trying to match everything inside the outer parentheses. — Raymond, Jul 21 '15 at 18:32
Are you wanting to modify the original text -- e.g., to `Transformer Winding Connections Wye (Star) or Delta`? — bdimag, Jul 21 '15 at 18:37

Avinash Raj · Accepted Answer · 2015-07-21T18:30:56.193

2

If you want to deal with only one level then this would be fine.

 @"\((?:\([^()]*\)|[^()])*\)"

or

If you don't want to match the outer paranthesis.

@"(?<=\()(?:\([^()]*\)|[^()])*(?=\))"

DEMO

edited Jul 21 '15 at 18:30

answered Jul 21 '15 at 18:25

Avinash Raj

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Wow, that second one worked perfectly! Thank you very much. – Raymond Jul 21 '15 at 18:38

score 1 · Answer 2 · answered Jul 21 '15 at 18:31

Here's the non-regex solution I mentioned in a comment, assuming your scenario is as simple as you laid out:

string longName = "Transformer Winding Connections (Wye (Star) or Delta)";

int openParenIndex = longName.IndexOf("(");
int closingParenIndex = longName.LastIndexOf(")");

if (openParenIndex == -1 || closingParenIndex == -1 
    || closingParenIndex < openParenIndex)
{
    // unexpected scenario...
}

string valueWithinFirstLastParens = longName.Substring(openParenIndex + 1, 
    closingParenIndex - openParenIndex - 1);

score 0 · Answer 3 · answered Jul 21 '15 at 18:31

Try this function, which doesn't use RegEx:

private static string RemoveOuterParenthesis(string str)
{
    int ndx = 0;
    int firstParenthesis = str.IndexOf("(", StringComparison.Ordinal);
    int balance = 1;
    int lastParenthesis = 0;

    while (ndx < str.Length)
    {
        if (ndx == firstParenthesis)
        {
            ndx++;
            continue;
        }
        if (str[ndx] == '(')
            balance++;
        if (str[ndx] == ')')
            balance--;
        if (balance == 0)
        {
            lastParenthesis = ndx;
            break;
        }
        ndx++;
    }

    return str.Remove(firstParenthesis, 1).Remove(lastParenthesis - 1, 1);
}

You'll want to clean it up a bit. Do some error checking. The functions assumes:

The string has parenthesis
The parenthesis are balanced
The string isn't null

Use Regex to remove outer parentheses from nested expression, but leave inner parentheses?

3 Answers3

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