php - Inserting number after alphanumeric group not followed by numeric group -

i have string contains 1 or more alphanumerical codes (starting letter) separated underscore. codes followed index number, separated underscore.

i want insert index number (always 1) after each code not followed index.

here's sample :

one                 =>  one_1 one_tw2_tre_for     =>  one_1_tw2_1_tre_1_for_1 one_tw2_tre_23_for  =>  one_1_tw2_1_tre_23_for_1 one_3_tw2_4_tre_45  =>  one_3_tw2_4_tre_45 

i can 2 calls preg_replace :

// add '_1' after each code $s = preg_replace('/[a-za-z][a-za-z0-9]+/', '$0_1', $s); // remove '_1' when followed index $s = preg_replace('/_1_([0-9]+)/', '_$1', $s); 

i'm wondering if there's way 2 1 preg_replace (i tried lookahead , lookbehind, without success) or different way might faster, processor wise.

thanks ! :-)

preg_replace('/[a-za-z][a-za-z0-9]+(?!_[0-9]|[a-za-z0-9])/', '$0_1', $s); 

or, more concise:

preg_replace('/[a-za-z][a-za-z0-9]+(?=_|$)(?!_[0-9])/', '$0_1', $s);