[英]Regex: possessive quantifier for the star repetition operator, i.e. \d**
From the GLib Reference Manual, section "Regular expression syntax" , subsection "Atomic grouping and possessive quantifiers": 在GLib参考手册的“正则表达式语法”部分 ,“原子分组和所有格修饰符”小节中:
Consider the pattern
\\d+foo
when applied to the string123456bar
: after matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \\d+ item, and then with 4, and so on, before ultimately failing.考虑将
\\d+foo
应用于字符串123456bar
:匹配所有6位数字,然后不匹配“ foo”后,匹配器的正常操作是仅使用5位数字匹配\\ d +项目,然后再尝试,然后4,依此类推,直到最终失败。If we use
(?>\\d+)foo
(called atomic grouping ) for the previous example, the matcher give up immediately on failing to match "foo" the first time.如果在前面的示例中使用
(?>\\d+)foo
(称为原子分组 ),则匹配器在第一次不匹配“ foo”时会立即放弃。When the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used:
\\d++foo
如上例所示,当原子组的子模式仅是一个重复项时,可以使用一种更简单的表示法,称为“可能量词”:
\\d++foo
My question is: is there any reason why there is no equivalent for the star ( *
) repetition operator? 我的问题是:星号(
*
)重复运算符没有等效的原因吗?
Example in Java: Java范例:
final String in = "123456";
// "plus" (+)
System.out.println(in.matches("\\d+")); // true
System.out.println(in.matches("(?>\\d+)")); // true
System.out.println(in.matches("\\d++")); // true
// "star" (*)
System.out.println(in.matches("\\d*")); // true
System.out.println(in.matches("(?>\\d*)")); // true
System.out.println(in.matches("\\d**")); // exception
The exception stack trace is: 异常堆栈跟踪为:
Exception in thread "main" java.util.regex.PatternSyntaxException: Dangling meta character '*' near index 3
\d**
^
at java.util.regex.Pattern.error(Pattern.java:1713)
at java.util.regex.Pattern.sequence(Pattern.java:1878)
at java.util.regex.Pattern.expr(Pattern.java:1752)
at java.util.regex.Pattern.compile(Pattern.java:1460)
at java.util.regex.Pattern.<init>(Pattern.java:1133)
at java.util.regex.Pattern.compile(Pattern.java:823)
at java.util.regex.Pattern.matches(Pattern.java:928)
at java.lang.String.matches(String.java:2090)
You can add +
to anything to make a possessive quantifier (it's not "doubling the quantifier"). 您可以在任何事物上加上
+
来构成所有格量词(不是“加倍量词”)。 So 所以
System.out.println(in.matches("\\d*+"));
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