I would like to implement a simple substitution cypher to mask private ids in URLs

I know how my IDs will look like (combination of upperchase ascii, digits and underscore), and they will be rather long, as they are composed keys. I would like to use a longer alphabet to shorten the resulting codes (I'd like to use upper and lower case ascii letters, digits and nothing else). So my incoming alphabet would be

[A-Z0-9_] (37 chars)

and my outgoing alphabet would be

[A-Za-z0-9] (62 chars)

so a compression of almost 50% would be available.

let's say my URLs look like this:

/my/page/GFZHFFFZFZTFZTF_24_F34

and I want them to look like this instead:

/my/page/Ft32zfegZFV5

Obviously both arrays would be shuffled to bring some random order in.

This does not have to be secure. if someone figures it out: fine, but I don't want the scheme to be obvious.

My desired solution would be to convert the string to an integer representation of radix 37, convert the radix to 62 and use the second alphabet to write out that number. is there any sample code available that does something similar? Integer.parseInt ( http://java.sun.com/javase/6/docs/api/java/lang/Integer.html#parseInt%28java.lang.String,%20int%29 ) has some similar logic, but it is hard-coded to use standard digit behavior

Any hints?

I am using java to implement this but code or pseudo-code in any other language is of course also helpful