正则表达式无法在Java代码中运行，尽管它可以在正则表达式网站上运行

Question

I guess I am just too tired, but I cannot figure it out after reading all the docs and tutorials. I have the following code:

import java.util.regex.*;

class Main {
  public static void main(String args[]) {
    String str = "ababaaabaaaaaabbbbbbaaaaaabaabababababaaaaaabbbbbbaaaaaabababababaaaaaabbbbbbaaaaaa";
    Pattern regex = Pattern.compile("(([ab])\2{5})(?!\1)(([ab])\4{5})(?!\3)(([ab])\6{5})");
    Matcher matcher = regex.matcher(str);

    int counter666 = 0;
    while (matcher.find()) ++counter666;
    System.out.println(counter666);
  }
}

I want to search the string for six occurrences of the same letter (should be a or b) three times after each other (but not three times the same letter). So, these are the only possible matches:

• "aaaaaabbbbbbaaaaaa"
• "bbbbbbaaaaaabbbbbb"

This regex is working on regex101 and on this page. However, my code always returns zero instead of three. Is there any option I have to set to make it working?

Answer 1

You're very very close. In order for the regular expression to work in a Java String you have to escape the \\ literals; otherwise the compiler doesn't put the value in the String (that you expect anyway). Like,

Pattern regex = Pattern.compile(
        "(([ab])\\2{5})(?!\\1)(([ab])\\4{5})(?!\\3)(([ab])\\6{5})");

And I get (with no other changes)

3

Answer 2

短版，步骤少，分为两组。

([ab])\\1{5}(?!\\1)([ab])\\2{5}(?!\\2)[ab]{6}