Asked  7 Months ago    Answers:  5   Viewed   36 times

In a PHP script, what regex should I use to check for mismatched parentheses in a string? Things that I want to allow include:

  • This is (ok)
  • This (is) (ok)

Things I want to prevent:

  • This is )bad(
  • This is also (bad
  • This is (bad (too)

Thanks!

Update: You guys all rock. Doing this with a regex seemed trickier than it should have, and these kinds of 2nd level answers are what makes stackoverflow beautiful. Thanks for the links and the pseudocode. I'm not sure who to give the answer to, so I apologize to everyone whose answers I can't accept.

 Answers

13

Regex is not the right tool for the job. Scan a string manually.

Pseudo-code:

depth = 0
for character in some_string:
    depth += character == '('
    depth -= character == ')'
    if depth < 0:
       break

if depth != 0:
   print "unmatched parentheses"
Wednesday, March 31, 2021
 
julesj
answered 7 Months ago
15

This will work only for non-nested parentheses:

    $regex = <<<HERE
    /  "  ( (?:[^"\\]++|\\.)*+ ) "
     | '  ( (?:[^'\\]++|\\.)*+ ) '
     | ( ( [^)]*                  ) )
     | [s,]+
    /x
    HERE;

    $tags = preg_split($regex, $str, -1,
                         PREG_SPLIT_NO_EMPTY
                       | PREG_SPLIT_DELIM_CAPTURE);

The ++ and *+ will consume as much as they can and give nothing back for backtracking. This technique is described in perlre(1) as the most efficient way to do this kind of matching.

Wednesday, March 31, 2021
 
KingCrunch
answered 7 Months ago
52

The standard disclaimer applies: Parsing HTML with regular expressions is not ideal. Success depends on the well-formedness of the input on a character-by-character level. If you cannot guarantee this, the regex will fail to do the Right Thing at some point.

Having said that:

<ab[^>]*>(.*?)</a>   // match group one will contain the link text
Saturday, May 29, 2021
 
lewiguez
answered 5 Months ago
33

You could use:

^(mailto|ftp|joe)

But to be honest, StartsWith is perfectly fine to here. You could rewrite it as follows:

string[] prefixes = { "http", "mailto", "joe" };
string s = "joe:bloggs";
bool result = prefixes.Any(prefix => s.StartsWith(prefix));

You could also look at the System.Uri class if you are parsing URIs.

Thursday, July 22, 2021
 
inieto
answered 3 Months ago
97

As Henning said, parser combinators would work for this. Here's an example using Parsec:

import Text.Parsec

grammar = many braces >> return ()
    where braces = choice [ between (char '(') (char ')') grammar
                          , between (char '[') (char ']') grammar
                          , between (char '{') (char '}') grammar
                          ]

isBalanced :: String -> Bool
isBalanced input = case parse (grammar >> eof) "" input of
                       Left  _ -> False
                       Right _ -> True
Saturday, October 9, 2021
 
Bono
answered 2 Weeks ago
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