How do I create a regular expression to capture {this} text and exclude \{that}?

Question

Hi Guys,

I need a regular expression to capture field names listed in a string. They are listed between Here are my requirements:

1. field names in curly braces
2. field names have no spaces
3. curly braces can be escaped with a \

So in the following:

capture {this} text and exclude \{that}?

The matches are {this} but not {that}.

I'm using PHP and preg_match (i could use a diff function, i'm open to ideas)

Any ideas? care to explain the result as well for me so I might learn something :)

Answer 1

You can use a negative lookbehind: (?<!\\)\{[^}]+\}. (Remember to escape the \ for PHP)

The expression (?<!something) is a zero-width assertion that will only allow the subsequent expression to match text that isn't preceded by something.
Thus, this regex matches \{[^}]+\} only if it's not preceded by \\. (The \ must be escaped)

Answer 2

$s = 'capture {this} text and exclude \{that} {a} \{b} {c}?';
preg_match_all('~(?<!\\\){[^}]+}~', $s, $matches);
var_dump($matches);

In details you can read about negative assertion (?<! here