c# regex with balancing groups not responding

Question

I have following code:

void Main()
{
  string template = @"
aaa 
{begin iteration items} 
  bbbbbb 
  {begin iteration subitems} 
    ccccccc 
  {end iteration subitems} 
  ddddddddd 
  {begin iteration items} 
    hhhhhhhhhhhhhhhhh
  {end iteration items} 
  iiiiiiiiiiiiiiiiiiiiiiiiiiii
{end iteration items} 
eeeeeeeeeeeeeeee
{begin iteration items} 
  ffffff
{end iteration items} 
gggggggggggg
  ";

  string re = @"
\{\s*begin\s+iteration\s+items\s*}
(?<template>
  (
    (?<iteration>\{\s*begin\s+iteration\s+items\s*})
    |(?<-iteration>\{\s*end\s+iteration\s+items\s*})
    |((?!(\{\s*begin\s+iteration\s+items\s*})|(\{\s*end\s+iteration\s+items\s*})).*?)
  )*(?(iteration)(?!))
)
\{\s*end\s+iteration\s+items\s*}
  ";

  Regex r = new Regex(re, RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace);
  var matches = r.Matches(template);
  matches.Dump();
}

When template is balanced then matches are returned and all is ok. But when I change {end iteration items} to {end1 iteration items} after iiiiiiiiiiiiiii line in template, then code stops to respond on matches.Dump() line (Dump() is an extension method to read/enumerate in LinQPad)

What is wrong? Is it possible to rewrite Regex so that it always respond?

EDIT My goal is to capture all top level <template> groups if syntax is valid, or capture nothing if not. I tried non-backtracking groups as Lucas adviced, but there no captures now when syntax is valid.

Answer 1

You're experiencing catastrophic backtracking here.

In short: a pattern in the form of ((something)*)* with nested quantifiers will trigger it, because the engine has to try all the possible combinations if a match can't be found right away.

You can use an atomic group to guard against it. The following should do the trick:

\{\s*begin\s+iteration\s+items\s*}
(?<template>
  (?>
    (?<iteration>\{\s*begin\s+iteration\s+items\s*})
    |(?<-iteration>\{\s*end\s+iteration\s+items\s*})
    |[^{]+
    |\{
  )*(?(iteration)(?!))
)
\{\s*end\s+iteration\s+items\s*}

Or use ((?>...)) instead of (?>...) if you need capturing.

I simplified the expression - there's no need for the lookahead anymore when using an atomic group, since these cases would be handled by the iteration groups. The last part of the alternative (\{) is here to account for lone opening braces, which are not part of the begin/end sequence. Most of the text is consumed by [^{]+ inside the atomic group, so backtracking cannot happen.