issue with find operation when searching character strings

Hi ,

I have a character string as for supoose

v_value  =  ' ABC DEFG HIJK JKLM NOPQ RST'

I want to search exact find for RST

I used if v_value CS 'RST' than do some operation 

but the problem is If I use if v_value CS 'RS' the if condition is satisfied.

Please suggest how can I achieve exact match .

Thanks,

RG