$re = '/(?<=[,\/])[A-Za-z]+_[^,\/]+/m';
$str = '/w_100/h_500/e_saturation:50,e_tint:red:blue/c_crop,a_100,l_text:Neucha_26_bold:Loremipsum./l_fetch:aHR0cDovL2Nsb3VkaW5hcnkuY29tL2ltYWdlcy9vbGRfbG9nby5wbmc/1488800313_DSC_0334__3_.JPG_mweubp.jpg';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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