$re = '/- .*?\[ACTIVE\] Execute(Thread:\s+\'[^\']*\').*-/m';
$str = '<2015-01-22 10:23:51,000> - <DEBUG> - <[ACTIVE] ExecuteThread: \'1\' for queue: \'weblogic.kernel.Default (self-tuning)\'> - MyLogClass - Message1
<2015-01-22 10:23:52,000> - <INFO> - <[ACTIVE] ExecuteThread: \'2\' for queue: \'weblogic.kernel.Default (self-tuning)\'> - MyLogClass - Message2
<2015-01-22 10:23:53,000> - <ERROR> - <[ACTIVE] ExecuteThread: \'2\' for queue: \'weblogic.kernel.Default (self-tuning)\'> - MyLogClass - Message3';
$subst = "\1";
$result = preg_replace($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