Match only @ and not @@ without negative lookbehind

Question

Using JavaScript, I need a regex that matches any instance of @{this-format} in any string. My original regex was the following:

@{[a-z-]*}

However, I also need a way to "escape" those instances. I want it so that if you add an extra @, the match gets escaped, like @@{this}.

I originally used a negative lookbehind:

(?



And that would work just fine, except... lookbehinds are an ECMAScript2018 feature, only supported by Chrome.

I read some people suggesting the usage of a negated character set. So my little regex became this:

(?:^|[^@])@{[a-z-]*}


...which would have worked just as well, except it doesn't work if you put two of these together: @{foo}@{bar}

So, anyone knows how can I achieve this? Remember that these conditions need to be met:


Find @{this} anywhere in a string
Be able to escape like @@{this}
Be able to put multiple adjacent, like @{these}@{two}
Lookbehinds must not be used

user_3541707 · Answer

If you include @@ in your regex pattern as an alternate match option, it will consume the @@ instead of allowing a match on the subsequent bracketed entity. Like this:

@@|(@{[a-z-]*})

You can then evaluate the inner match object in javascript. Here is a jsfiddle to demonstrate, using the following code.

var targetText = '@{foo} in a @{bar} for a @@{foo} and @{foo}@{bar} things.'
var reg = /@@|(@{[a-z-]*})/g;
var result;
while((result = reg.exec(targetText)) !== null) {
        if (result[1] !== undefined) {
        alert(result[1]);
    }
}

user_9515207 · Answer

You could use (?:^|[^@])@ to match the start of the pattern, and capture the following @{} in a group. Since you don't want the initial (possible) [^@] to be in the result, you'll have to iterate over the matches manually and extract the group that contains the substring you want. For example:

function test(str) {
  const re = /(?=(?:^|[^@])(@{[a-z-]*}))./g;
  let match;
  const matches = [];
  while (match = re.exec(str)) {
    matches.push(match[1]); // extract the captured group
  }
  return matches;
}
console.log(test('@@{this}'))
console.log(test('@{these}@{two}'))

Match only @ and not @@ without negative lookbehind

2 Answers