regexp中的(*)和.*有什么区别? [英] What is the difference between (*) and .* in regexp?

问题描述

.*是零个或多个次数的任何字符.我试图找到以元音开头的单词，我用过

.* is any character zero or more number of times. I was trying to find words starting with vowel and i used

^[aeiou](*) 

它给了我所有以元音开头的单词.当我执行^[aeiou].*时，给出相同的结果 现在我正在寻找以元音结尾的单词.我做到了

And it gave me all words starting with vowel. Same result is given when i do ^[aeiou].* Now i was looking for words that end with vowels. I did

^(*)[aeiou]$ 

它没有给出结果，但是当我执行^.*[aeiou]$时它给出了有效的结果.请解释两者含义上的差异.谢谢

It gave no result but when i did ^.*[aeiou]$ it gave valid results. Please explain the difference in the meaning of both. Thanks

推荐答案

看来，在Oracle 11中，*和+模式(之前没有任何内容)会生成零宽度的匹配项.因此(*)和(+)捕获的组()分别包含零宽度模式*或+.

It appears that, in Oracle 11, the * and + patterns (with nothing preceding them) generate zero-width matches. So (*) and (+) are capturing groups () containing the zero-width patterns * or + respectively.

所有这些查询都返回一行:

All these queries return a row:

SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '*' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '(*)' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^*' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^(*)' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '*1' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '+' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '(+)' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^+' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^(+)' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '+1' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^+1' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '+1$' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^+1$' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '()' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '12', '1(*)2' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '12', '1(+)2' );

这些查询不返回任何行:

These queries do not return any rows:

SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '*2' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^*$' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', '^+$' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '12', '11(*)2' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '12', '11(+)2' );

此外，将任何模式与NULL匹配或将字符串与NULL模式匹配都不会返回任何行:

Also, matching any pattern against NULL or a string against a NULL pattern will not return any rows:

SELECT * FROM DUAL WHERE REGEXP_LIKE( NULL, '*' );
SELECT * FROM DUAL WHERE REGEXP_LIKE( '1', NULL );

使用REGEXP_SUBSTR:

SELECT REGEXP_SUBSTR( '1', '+' ) FROM DUAL;

输出包含NULL的单行.

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