Java regex alternation operator “|” behavior seems broken

问题

Trying to write a regex matcher for roman numerals. In sed (which I think is considered 'standard' for regex?), if you have multiple options delimited by the alternation operator, it will match the longest. Namely, "I|II|III|IV" will match "IV" for "IV" and "III" for "III"

In Java, the same pattern matches "I" for "IV" and "I" for "III". Turns out Java chooses between alternation matches left-to-right; that is, because "I" appears before "III" in the regex, it matches. If I change the regex to "IV|III|II|I", the behavior is corrected, but this obviously isn't a solution in general.

Is there a way to make Java choose the longest match out of an alternation group, instead of choosing the 'first'?

A code sample for clarity:

public static void main(String[] args)
{
    Pattern p = Pattern.compile("six|sixty");
    Matcher m = p.matcher("The year was nineteen sixty five.");
    if (m.find())
    {
        System.out.println(m.group());
    }
    else
    {
        System.out.println("wtf?");
    }
}

This outputs "six"

回答1:

No, it's behaving correctly. Java uses an NFA, or regex-directed flavor, like Perl, .NET, JavaScript, etc., and unlike sed, grep, or awk. An alternation is expected to quit as soon as one of the alternatives matches, not hold out for the longest match.

You can force it to continue by adding a condition after the alternation that can't be met until the whole token has been consumed. What that condition might be depends on the context; the simplest option would be an anchor ($) or a word boundary (\b).

"\\b(I|II|III|IV)\\b"

EDIT: I should mention that, while grep, sed, awk and others traditionally use text-directed (or DFA) engines, you can also find versions of some of them that use NFA engines, or even hybrids of the two.

回答2:

I think a pattern that will work is something like

IV|I{1,3}

See the "greedy quantifiers" section at http://download.oracle.com/javase/1.4.2/docs/api/java/util/regex/Pattern.html

Edit: in response to your comment, I think the general problem is that you keep using alternation when it is not the right thing to use. In your new example, you are trying to match "six" or "sixty"; the right pattern to use is six(ty)?, not six|sixty. In general, if you ever have two members of an alternation group such that one is a prefix of another, you should rewrite the regular expression to eliminate it. Otherwise, you can't really complain that the engine is doing the wrong thing, since the semantics of alternation don't say anything about a longest match.

Edit 2: the literal answer to your question is no, it can't be forced (and my commentary is that you shouldn't ever need this behavior).

Edit 3: thinking more about the subject, it occurred to me that an alternation pattern where one string is the prefix of another is undesirable for another reason; namely, it will be slower unless the underlying automaton is constructed to take prefixes into account (and given that Java picks the first match in the pattern, I would guess that this is not the case).

来源：https://stackoverflow.com/questions/4515309/java-regex-alternation-operator-behavior-seems-broken

标签

java

regex

regex-alternation