Introduction to Computational Linguistics

11. Interlude: Regular Expressions in OCaML 35
the position in the string where the search should start. The result is an integer
and it gives the initial position of the next string that falls under the regular expression.
If none can be found, OCaML raises the exception Not_found. If you
want to avoid getting the exception you can also use string_match before to see
whether or not there is any string of that sort. It is actually superfluous to first
check to see whether a match exists and then actually performing the search. This
requires to search the same string twice. Rather, there are two useful functions,
match_beginning and match_end, which return the initial position (final position)
of the string that was found to match the regular expression. Notice that you
have to say Str.match_beginning(), supplying an empty parenthesis.
There is an interesting difference in the way matching can be done. The Unix
matcher tries to match the longest possible substring against the regular expression
(this is called greedy matching), while the Windows matcher looks for the just
enough of the string to get a match (lazy matching). Say we are looking for a ∗ in
the string bcagaa.
(98)
let ast = Str.regexp "a*";;
Str.string_match ast "bcagaa" 0;;
In both versions, the answer is ”yes”. Moreover, if we ask for the first and the last
position, both version will give 0 and 0. This is because there is no way to match
more than zero a at that point. Now try instead
(99) Str.string_match ast "bcagaa" 2;;
Here, the two algorithms yield different values. The greedy algorithm will say that
it has found a match between 2 and 3, since it was able to match one successive
a at that point. The lazy algorithm is content with the empty string. The empty
string matches, so it does not look further. Notice that the greedy algorithm does
not look for the longest occurrence in the entire string. Rather, it proceeds from
the starting position and tries to match from the successive positions against the
regular expression. If it cannot match at all, it proceeds to the next position. If it
can match, however, it will take the longest possible string that matches.
The greedy algorithm actually allows to check whether a string falls under a