PHP preg_match function

i want to parse the string "_0_ _1_ _2_ _3_" to get the numbers in an array

i could do that for a fixed string but if the string is a variable in length i can't figure out how to do it but this won't repeat it self i.e it can't do the job if the string becomes "_0_ _1_ _2_ _3_ _4_ _5_" i need help