Extraction of a substring

We are using Version 5.0.5.7 for our data analysis.

I do now have the following problem extracting data from a file:

Using fread I read a line from an ASCII file into a string varibale. The
string then contains various characters (letters numbers, special
characters) of which only a substring contains the information needed, in
this special case this is the last 6 characters.

the reading part of my procedure looks as follows:

open/R/P=DATA refnum as  filename
for(i=0;i<17;i+=1)
    FReadLine refNum, help_str  //steps through the lines, information wanted is in line 19(counting from1)
endfor
    FReadLine refNum, help_str
    //now help_str contains the line cotaining the needed numbers
       //s.th. like "shajshdja : 84298  / smasjdak :jkahsdf :123456"
      //"123456" is the substring I want to extract into a new string and its allways at the end of the line

close refnum

I can locate the string I am looking for using strsearch. So I know that
in one particular case it starts with the 85th character of the total string
and is 6 characters long.

Can I somehow extract a substring from the read-in string by using its
length and position? I can not use stringbykey or listitem as the sting is
quite unstructured. But is there a possibilty using sscanf?