Question

Say i have some rules and actions, like so

,                   { do something; }
\.                  { do_something; }
\(                  { do_someting; }
\)                  { do_something); }
:                   { do_something; }
...

Whenever i match the a rule on the left, the action on the right gets called.
But what if i want to match everything else ? How could i do that?

using something like

.*          { do_something; }

won’t work because it also applies to the above rules.

Answer 1

According to the lex spec:

During pattern matching, lex shall search the set of patterns for the
single longest possible match. Among rules that match the same number
of characters, the rule given first shall be chosen.

So I would go with:

.  { do_something; }

…and put it as the last rule.

This will match a single character, except for newline (“A <newline> shall not be matched by a period operator”). If you also want to match newline:

.|\n  { do_something }

Finally, if you do not actually want to consume the character, you need an extended regular expression (ERE) that matches the empty string. You could try:

.{0}  { do_something }

As I read the spec for lex and EREs, I believe that should work, but it would not surprise me if it tickled some bug somewhere 🙂