Why isn't the “X, at MOST n times” regex quantifier included in Java, in the “X{,n}” syntax? [closed]

问题

Java docs tell us you can have this greedy quantifiers:

X{n}    X, exactly n times
X{n,}   X, at least n times
X{n,m}  X, at least n but not more than m times

There is no mention of having X, at MOST n times.

So I made a little test:

boolean atMost = Pattern.matches("X{,3}", "XX");

I expected atMost to be true, since it's safe to assume the lower bound to be zero. Instead, I got an exception:

Exception in thread "main" java.util.regex.PatternSyntaxException: Illegal repetition near index 0
X{,3}
^

Note it's not a thing of being Greedy, Reluctant or Possessive: "X{,3}?" and "X{,3}+" also don't work.

I could have used "X{0,3}" to achieve the pattern I'm looking for, but that's not my question.

Why isn't the X, at MOST n times regex quantifier included in Java, in the X{,n} syntax?

How is it like in other programming languages or regex "flavors"?

回答1:

Its not a {n,m} issue, its a PARSING issue.

Left to right, { + number is all that has to be validated for a possible range
quantifier, otherwise { is literal and the parse continues.

Also, the paser wants a simple number available as the range start to compare with the
range end. It doesn't want to dither/allow simultaneous {min_default,max_default} or {,}

It would be too complicated at that low level to allow both.

来源：https://stackoverflow.com/questions/22079519/why-isnt-the-x-at-most-n-times-regex-quantifier-included-in-java-in-the-x

标签

java

regex

quantifiers