I have a string of the following form
$string = "This is {test} for [a]{test2} for {test3}.";
I want to get all curly brackets that are not prefixed by square brackets. Thus, in the above string I would like to get {test} and {test3} but not [a]{test2}.
I found in the answer https://stackoverflow.com/a/977294/2311074 that this might be possible with negative lookahead. So I tried
$regex = '/(?:(?!\[[^\}]+\])\{[^\}]+\})/';
echo preg_match_all($regex, $string, $matches) . '<br>';
print_r($matches);
but this still gives me all three curly brackets.
3
Array ( [0] => Array ( [0] => {test} [1] => {test2} [2] => {test3} ) )
Why is this not working?
The reason your regex fails is that it matches any { (followed with 1+ non-}s and then a }) if it does not start a sequence of the patterns inside the negative lookahead, a [, 1+ chars other than } and then a ] (and it is always true, so, you get all {...} substrings as a result).
\[[^]]*]\{[^}]+}(*SKIP)(*F)|\{[^\}]+}
See the regex demo.
Details:
\[[^]]*]\{[^}]+}(*SKIP)(*F) - matches\[ - a [[^]]* - 0+ chars other than ]]\{ - ]{ substring[^}]+ - 1+ chars other than ]} - a literal }(*SKIP)(*F) - PCRE verbs discarding the text matched so far and forcing the engine to go on looking for the next match from the current position (as if a match occurred)| - or\{[^\}]+}:\{ - a {[^\}]+ - 1+ chars other than } and} - a literal }.See the PHP demo:
$string = "This is {test} for [a]{test2} for {test3}.";
$regex = '/\[[^]]*]\{[^}]+}(*SKIP)(*F)|\{[^}]+}/';
echo preg_match_all($regex, $string, $matches) . "
";
print_r($matches[0]);
Output:
2
Array
(
[0] => {test}
[1] => {test3}
)
If you are sure opening curly braces would only be preceded with a pair of square brackets (balanced) then a negative lookbehind will do the job:
(?<!]){[^}]*}