Picking a character from a word

[Guy Harris]

>    Unfortunately, "expr substr" is a BSD-ism which has yet to find its way
>    into the SVID.

> Or the manual entry.

Actually, "expr substr" is an AT&T-ism that they never documented and that they
thought better of in S3 or so and deleted.  It has the disadvantage that a
token of "substr" - or "index" or "length" - is always treated as the operator
in question, even if it's supposed to be just a string.  (Another undocumented
feature of V7's "expr" - namely the "match" prefix operator which is equivalent
to the ":" infix operator - is still around in S5, so it doesn't work as a
string.)