$re = '/weapon(?:(?:_[^\W_]+)+(?=_[^\W_]{2}_)|(?:(?!_[^\W_]{2}_)_[^\W_]+)+)/';
$str = 'econ/default_generated/weapon_knife_flip_so_night_medium
econ/default_generated/weapon_bayonet_hy_ocean_knife_medium
econ/default_generated/weapon_knife_gut_aq_blued_heavy
econ/default_generated/weapon_knife_karambit_hy_webs_medium
econ/default_generated/weapon_knife_m9_bayonet_so_night_lightasd
econ/default_generated/weapon_knife_tactical_hy_ddpat_light
econ/default_generated/weapon_knife_falchion_aa_fade_heavy
econ/default_generated/weapon_knife_survival_bowie_am_doppler_phase3_heavy
econ/default_generated/weapon_knife_butterfly_hy_ddpat_medium
econ/default_generated/weapon_knife_push_hy_ddpat_light';
preg_match_all($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