I'm currently reading the documentation on Lex written by Lesk and Schmidt, and get confused by the REJECT action.
Consider the two rules
a[bc]+ { ... ; REJECT;}
a[cd]+ { ... ; REJECT;}
Input:
ab
Only the first matches, and see what we get from the material.
The action REJECT means ``go do the next alternative.'' It causes whatever rule was second choice after the current rule to be executed.
However, there is no second choice, will there comes a error?
There are really very few use cases for REJECT; I don't think I've ever seen an instance of it in use other than in examples.
Anyway, unless you specify %option nodefault (or the -s command-line flag), flex will add a default fallback action to your ruleset, equivalent to
.|\n ECHO;
In your case, that pattern will match after the REJECT.
However, it is possible to override the default action; for example, you could add the rule:
.|\n REJECT;
In that case, flex really will not have an alternative after the two REJECTs, and it will print an error message on stderr ("flex scanner jammed") and then call exit.