How do I match any string not matched in the preceding rules?

One way to assign precedence, is to place the more specific rules first. If two rules would match the same input (same sequence of characters) then the first rule listed in the flex input wins, e.g.,

%%
foo[a-zA-Z_]+    return FOO_ID;
bar[a-zA-Z_]+    return BAR_ID;
[a-zA-Z_]+       return GENERIC_ID;

Note that the rule [a-zA-Z_]+ must come *after* the others. It will match the same amount of text as the more specific rules, and in that case the flex scanner will pick the first rule listed in your scanner as the one to match.