Returns a substring of a string contained in a memory variable; optionally,
modifies the memory variable by removing the substring.
<string> // A quoted name of a memory variable.
, <start pos> // 1 to n.
, <num> // A number. See Notes.
, <mode> // Any combination of L and R and U. See notes.
See also StrExtract. StrExtract_Var will handle very long strings.
If <mode> contains "L", then <num> is interpreted as a length; otherwise, a position.
If <mode> contains "R", then operations are performed right-to-left; otherwise, left to
If <mode> contains "U", then the substring is removed from the memory variable;
otherwise, no modification occurs.
Assume contents of mvXX are "FileExpress". In the example the <start pos> (4) is counted from the right, so "r" is the starting position in the string. <num> is interpreted as a length (3), moving to the left, so "x" is the end position. The substring returned is "xpr". This substring is removed from the source.
Returns: "xpr"; mvXX is now "Fileess".