st: extract portion of variable name to use in variable label

[Andy Baxter <andrew.baxter@cms.k12.nc.us>]

I have a large data set in which the variable names reflect the subject 
and metric of a specific test, e.g., a_alg1_ss is the "Algebra 1 Scale 
Score."
All of the variable names have a common structure:  a prefix of a_, a 
test identifier string, and a suffix of _[metric of test, e.g., al for 
achievement level or ss for scale score].
The current variable labels reflect the test metric, but not the test 
identifier.

Here is a subset of the variables:

. describe  a_alg1_ss   a_alg1_al a_alg1rt1_ss   a_alg1rt1_al

             storage  display     value
variable name   type   format      label      variable label
-------------------------------------------------------------------------------
a_alg1_ss       int    %8.0g                   Scalescore
a_alg1_al       byte   %8.0g                   Achievement Level
a_alg1rt1_ss    int    %8.0g                   Scalescore
a_alg1rt1_al    byte   %8.0g                   Achievement Level

I am trying to write syntax that will extract the test identifier from 
the variable name and insert it into the variable label.  I would like 
to wind up with:

variable name                     variable label
a_alg1_ss                           alg1 Scalescore
a_alg1_al                           alg1 Achievement Level
a_alg1rt1_ss                       alg1rt Scalescore
a_alg1rt1_al                        alg1rt Achievement Level

My Stata version is:
.about
Stata/SE 9.2 for Windows
Born 20 Jul 2007

Thanks for your consideration.
--
Andy Baxter
Center for Research & Evaluation
Charlotte-Mecklenburg Schools
980.343.1603 (phone)
980.343.6660 (fax)



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