如何使用正则表达式实现递归回文检查器？

So far I've been using ^[a-zA-Z]+( [a-zA-z]+)*$ to make sure user input Has no space in the beginning and the end and not to accept numbers or special characters and only to accept alphabetical characters.

到目前为止,我一直在使用^ [a-zA-Z] +([a-zA-z] +)* $来确保用户输入在开头和结尾没有空格而不接受数字或特殊字符并且只接受字母字符。

I looked at the regex list online and in my texts I can't seem to formulate one under these conditions for a recursive palindrome program:

我在网上查看了正则表达式列表,在我的文本中,我无法在这些条件下为递归回文程序制定一个:

• Accepts strings containing upper case or lower case characters.

接受包含大写或小写字符的字符串。

• Accepts punctuation, and single spaced blanks.

接受标点符号和单行间隔空格。

I don't think I'll need a regex for the following after validation, but if there is I'd like to know what it is.

在验证后我不认为我需要一个正则表达式,但如果有,我想知道它是什么。

• After validation upper case letters must be converted to lower case.

验证后,大写字母必须转换为小写字母。

• The punctuation and spaces are to be ignored.

标点符号和空格将被忽略。

1 个解决方案

#1

If I understand you correctly, you want to create a recursive palindrome checker that employs regular expressions, in Java. I'm interested in learning Java, so I gave it a shot as my own sort of "homework problem," although it is probably yours too.

如果我理解正确,你想在Java中创建一个使用正则表达式的递归回文检查器。我对学习Java感兴趣,所以我把它作为我自己的“家庭作业问题”,但它也可能是你的。

import java.lang.*;
import java.util.*;
import java.util.regex.*;

class Main
{
    public static boolean recursivePalindrome(String str)
    {
        // We need two patterns: one that checks the degenerate solution (a
        // string with zero or one [a-z]) and one that checks that the first and
        // last [a-z] characters are the same. To avoid compiling these two
        // patterns at every level of recursion, we compile them once here and
        // pass them down thereafter.
        Pattern degeneratePalindrome = Pattern.compile("^[^a-z]*[a-z]?[^a-z]*$", Pattern.CASE_INSENSITIVE | Pattern.DOTALL);
        Pattern potentialPalindrome  = Pattern.compile("^[^a-z]*([a-z])(.*)\\1[^a-z]*$", Pattern.CASE_INSENSITIVE | Pattern.DOTALL);
        return recursivePalindrome(str, degeneratePalindrome, potentialPalindrome);
    }

    public static boolean recursivePalindrome(String str, Pattern d, Pattern p)
    {
        // Check for a degenerate palindrome.
        if (d.matcher(str).find()) return true;

        Matcher m = p.matcher(str);

        // Check whether the first and last [a-z] characters match.
        if (!m.find()) return false;

        // If they do, recurse using the characters captured between.
        return recursivePalindrome(m.group(2), d, p);
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        String str1 = "A man, a plan, a canal... Panama!";
        String str2 = "A man, a pan, a canal... Panama!";

        System.out.println(str1 + " : " + Boolean.toString(recursivePalindrome(str1)));
        System.out.println(str2 + " : " + Boolean.toString(recursivePalindrome(str2)));
    }
}

The output is:

输出是:

A man, a plan, a canal... Panama! : true
A man, a pan, a canal... Panama! : false