concat for pgsql?

i'm still porting my app from mysql to pgsql. i'm stuck on this one. in my
mysql app where i have a tree structure defined as:

01
0101
0102
...
ZZ01
ZZ02
ZZ0301
...etc


it's basically base 36

when i would move a node in the tree, i would do something like this in
mysql:

UPDATE tbl SET code =
CONCAT('NEWPREFIX',SUBSTRING(code,LENGTH('parent_code'))) WHERE \
code LIKE 'parent_code%';

so if i moved all the nodes under 01010201 to AA, it would UPDATE all the
paths starting with 01010201 with AA using CONCAT. i can't seem to find a
similar function in pgsql. ofcourse i could pull the results into an array
in php and loop through it to make the changes, and UPDATE them
individually back to pgsql, but that's a very slow process.