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Rope Language Reference - "substr".

The "substr" action returns a specified portion of an original string (refer to string for comments on Rope's string handling). "substr" takes three arguments (in the following order) ..

the original string
the offset at which the sub-string is to start
the desired sub-string length.

For example, this script prints the word "Three".

put( "Three blind mice" $a )
substr( $a 0 5 ) println

And this prints "bli"

put( "Three blind mice" $a )
substr( $a 6 3 )

Note:

It is an error for either the offset or length to be negative
Offset zero refers to the first character in the string, and offset one refers to the second (etc), as per the conventions of C, Java, Perl - etc.
If the "offset" is beyond the end of the string, the result will silently (ie: no error is reported) be an empty return string.
If the "length + offset" is greater than the length of the string, then the return will be shorter than the specified length.
Unlike the "substr" function in some other languages, the Rope version requires all three arguments.