In the name of POSIX compliance, flex supports a translation
table for mapping input characters into groups. The table
is specified in the first section, and its format looks
like:
%t 1 abcd 2 ABCDEFGHIJKLMNOPQRSTUVWXYZ 52 0123456789 6 \t\ \n %t
This example specifies that the characters `a', `b', `c',
and `d' are to all be lumped into group #1, upper-case letters in
group #2, digits in group #52, tabs, blanks, and newlines into group #6,
and no other characters will appear in the patterns. The group numbers
are actually disregarded by flex; %t serves, though, to
lump characters together. Given the above table, for example, the
pattern `a(AA)*5' is equivalent to `d(ZQ)*0'. They both say,
"match any character in group #1, followed by zero or more pairs of
characters from group #2, followed by a character from group #52."
Thus `%t' provides a crude way for introducing equivalence classes
into the scanner specification.
Note that the `-i' option (see below) coupled with the
equivalence classes which flex automatically generates take
care of virtually all the instances when one might consider
using `%t'. But what the hell, it's there if you want it.