$re = '/;(?=[^[]*])/m';
$str = '["Text1;Text2;....;TextN"];asjkdjksd;ajksdjksad["Text1;Text2;....;TextN"]
.["Text1;Text2"]...long text...["Text1;Text2;Text3"]....long text...["Text1;...;TextN"]...long text...
I ONLY want to change the ";" in the ["Text1;...;TextN"]';
$subst = ",";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php