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How can I make REJECT cascade across start condition boundaries?

You can do this as follows. Suppose you have a start condition ‘A’, and after exhausting all of the possible matches in ‘<A>’, you want to try matches in ‘<INITIAL>’. Then you could use the following:

%x A
<A>rule_that_is_long    ...; REJECT;
<A>rule                 ...; REJECT; /* shorter rule */
<A>.|\n  {
/* Shortest and last rule in <A>, so
* cascaded REJECTs will eventually
* wind up matching this rule.  We want
* to now switch to the initial state
* and try matching from there instead.
yyless(0);    /* put back matched text */