PHP Regex: not wrapped items

Question

How i can find and replace all non-wrapped items in string via PHP regular expression?

For example, I have source string "2a{2}b2ac1{1}a{2}aab12{1}b2a{1}2" and try find symbol "2", which are not covered by "{" and "}", after this replace it with "{3}":

$input_lines = "2a{2}b2ac1{1}a{2}aab12{1}b2a{1}2";
$regex = "/[^\{](2)[^\}]/";
$input_lines = preg_replace("/[^\{](2)[^\}]/", "{3}", $input_lines);
echo $input_lines;
// 2a{2}{3}c1{1}a{2}aab{3}1}{3}{1{3}

How you can see, it's now work :(

Answer 1

您可以尝试以下方法：

$input_lines = preg_replace('/(?<!{)2(?!})/', '{3}', $input_lines);

Answer 2

/(?<!{)2(?!})/ will do the trick. Updated to take care of scenarios like 123} .

$input_lines = '2a{2}b2ac1{1}a{2}aab12{1}b2a{1}2';
$input_lines = preg_replace('/(?<!{)2(?!})/', '{3}', $input_lines);

var_dump($input_lines);
// string(42) "{3}a{2}b{3}ac1{1}a{2}aab1{3}{1}b{3}a{1}{3}"

Explanation:

/                                 # Beginning delimiter
 (?<!{)                           # Lookbehind for anything other than {
  2                               # Match 2
 (?!})                            # Lookahead for anything other than }
/                                 # Ending delimiter