predefined tag extraction to generate random value with php

i was looking for an easy way to generate random value with predefined tag for example

[5,7,aAn]

i would like to extract this from text area, and replace it with random min would be 5 & max would be 7, and each time i can play with

"a" : lower case;
"A" : upper case;
"n" : numbers;

Is it possible to extract these value and use them with php to generate the predefined random. Thank you