Does Perl currently (5.8 and 5.10) make any promises about the order alternations will be used?

Question

Given an alternation like /(foo|foobar|foobaz)/ does Perl 5.8 or 5.10 make any promises about which of the three will be used first, and if it does where in the documentation does it make that promise?

See the related question Does Perl 6 make any promises about the order alternations will be used?

Answer 1

There seems to be a promise made in perldoc perlrequick:

To match dog or cat , we form the regex dog|cat . As before, perl will try to match the regex at the earliest possible point in the string. At each character position, perl will first try to match the first alternative, dog . If dog doesn't match, perl will then try the next alternative, cat . If cat doesn't match either, then the match fails and perl moves to the next position in the string.

perldoc perlretut seems to make the promise in an even stronger way (but with a caveat):

"cats"          =~ /c|ca|cat|cats/; # matches "c"
"cats"          =~ /cats|cat|ca|c/; # matches "cats"

Here, all the alternatives match at the first string position, so the first alternative is the one that matches. If some of the alternatives are truncations of the others, put the longest ones first to give them a chance to match.

"cab" =~ /a|b|c/ # matches "c"
                 # /a|b|c/ == /[abc]/

The last example points out that character classes are like alternations of characters. At a given character position, the first alternative that allows the regexp match to succeed will be the one that matches.

Answer 2

http://perldoc.perl.org/perlre.html#Combining-RE-Pieces:

if we match a regular expression a|ab against "abc" , will it match substring "a" or "ab" ? One way to describe which substring is actually matched is the concept of backtracking (see "Backtracking"). However, this description is too low-level and makes you think in terms of a particular implementation.

Another description starts with notions of "better"/"worse"

Again, for elementary pieces there is no such question, since at most one match at a given position is possible. This section describes the notion of better/worse for combining operators. In the description below S and T are regular subexpressions.

...

• S|T

When S can match, it is a better match than when only T can match.

(In context, this is qualified to only when the match using S matches at least as early in the string as that using T.)

Answer 3

In general, the default regex engine in Perl tries to make the leftest and longest match, in that order. If it can match the leftmost alternation possibility and still satisfy the rest of the regex it will.

However, you can change the regex engine.