<?php
$search_field_pattern = "/\bdata\[SearchForm]\[(\w+)]/i";
if(preg_match($search_field_pattern, "data[SearchForm][somestringhere]", $m)) { echo $m[1];
}
PD9waHAKCiRzZWFyY2hfZmllbGRfcGF0dGVybiAgPSAiL1xiZGF0YVxbU2VhcmNoRm9ybV1cWyhcdyspXS9pIjsKaWYocHJlZ19tYXRjaCgkc2VhcmNoX2ZpZWxkX3BhdHRlcm4sICJkYXRhW1NlYXJjaEZvcm1dW3NvbWVzdHJpbmdoZXJlXSIsICRtKSkgewogICAgZWNobyAkbVsxXTsKfQ==