Understanding regex in java

Question

My program works how I want it to but I stumbled upon something that I don't understand.

String problem = "4 - 2";
problem = problem.replaceAll("[^-?+?0-9]+", " ");
System.out.println(Arrays.asList(problem.trim().split(" ")));

prints [4, -, 2]

but

String problem = "4 - 2";
problem = problem.replaceAll("[^+?-?0-9]+", " ");
System.out.println(Arrays.asList(problem.trim().split(" ")));

doesn't even do anything with the minus sign and prints [4, 2]

Why does it do that, it seems like both should work.

I've found regex debugging webapps like debuggex.com very useful. — lexicore
– lexicore, Commented Dec 26, 2016 at 20:14

Casimir et Hippolyte · Accepted Answer · 2016-12-26 19:35:21Z

3

The hyphen has a special meaning inside a character class: it is used to define a character range (like a-z or 0-9), except when:

it is at the start of the character class or immediately after the negation character ^
it is escaped with a backslash
it is at the end of the character class
with some regex engines when it is after a shorthand character class like \w, \s, \d, \p{thing},... (for these one, the situation isn't ambiguous, it can't be a range)

In the first example, it is seen as a literal hyphen (since it is at the beginning).

In your second example, I assume that ?-? defines a range between ? and ? (that is nothing more than the character ?)

Note: ? doesn't have a special meaning inside a character class (it's no more a quantifier but a simple literal character)

edited Dec 26, 2016 at 19:35

answered Dec 26, 2016 at 19:24

Casimir et Hippolyte

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Sualeh Fatehi · Accepted Answer · 2016-12-26 19:24:58Z

0

If you are trying to match a literal - inside of a [ and ], it must be escaped, \-. In the first case, ^ marks the beginning of a match, so really you are match -?, so there is nothing to escape. In the second case, it seems like you are matching ?-?, which can cause the regular expression to function in a way you did not expect.

PS: To escape in Java, you need \\ instead of \.

answered Dec 26, 2016 at 19:24

Sualeh Fatehi

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yshavit · Accepted Answer · 2016-12-26 19:29:58Z

0

In the second example, +?-? means "a plus sign, or any chars between ? and ?, inclusive. Of course, that means just ?, so the whole regex is equivalent to [^+?0-9]+.

The only time within a character class (between the square brackets) that - doesn't mean "between, inclusive" is at the start of the character class, or immediately following a ^ that starts it, or at the end of the character class, or when it's escaped (\-).

edited Dec 26, 2016 at 19:29

answered Dec 26, 2016 at 19:24

yshavit

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