[R] regular expression question

[markleeds at verizon.net]

I am very much a novice with regular expressions and I have spent
a decent amount of time trying to do the following and I can't
get it.

i have variables that are of type character but
they have number characters at the end. for example :

"AAL123"
"XELB245"
"A247"

I want a command that gives me just gives me the letter characters
for each one. 
the letter characters always start first and then the number characters come second and it never flips back to letter characters
once the number characters start. i am using R-2.20 on
windows Xp. Thanks. substring doesn't work because the
length of the letter characters can vary.


always occur first with the numbers starting