CODEWITHSUNDEEP
javaregexstringalgorithmreverse
Chris Smith
Chris Smith

Reputation: 409

Java: How to reverse string based on non alphabetic delimiters?

How to reverse string based on non alphabetic delimiters? I suspect my regex may be the problem. 

String fileContent = "Joe'); MAKE TEST random;--";
String[] splitWords = fileContent.split("[^a-zA-Z0-9']+");

StringBuilder stringBuilder = new StringBuilder();
for (String word : splitWords) {
    int idx = fileContent.indexOf(word, stringBuilder.length());
    String delim = fileContent.substring(stringBuilder.length(), idx);
    stringBuilder.append(delim);

    StringBuilder output = new StringBuilder(word).reverse();
    stringBuilder.append(output);
}
return stringBuilder.toString();

Current output: 'eoJ); EKAM TSET modnar

Desired output: eoJ'); EKAM TSET modnar;--

Upvotes: 0

Views: 97

Answers (3)

SomeDude
SomeDude

Reputation: 14238

You don't need a regex for this. It seems you want to reverse characters that are only alphabets or digits. Then you could do this way - get the start and end indices of the character array where you find the character to be a letter or digit and then reverse in place. Then return a new String with the characters reversed in place.

private static void reverseWords(char[] c) {
    int start = 0, end = c.length;
    while ( start < end ) {
      int pre = start;
      while ( start < c.length && Character.isLetterOrDigit(c[start]) ) 
        start++;
      if ( pre < start ) 
         reverseWord(c, pre, start-1);
      start++;  
    }
  }

  private static void reverseWord(char[] c, int start, int end) {
    while ( start < end ) {
      char temp = c[start];
      c[start] = c[end];
       c[end]  = temp;
      start++;
      end--;
    }
  }

You can test this code here

Upvotes: 2

Syed Afzal
Syed Afzal

Reputation: 269

Your code works with two changes:

  1. Replace the pattern
  2. Test for trailing delimiter

String fileContent = "Joe'); MAKE TEST random;--"; String[] splitWords = fileContent.split("\W"); // W is non word character or I forgot

StringBuilder stringBuilder = new StringBuilder();
for (String word : splitWords) {
    int idx = fileContent.indexOf(word, stringBuilder.length());
    String delim = fileContent.substring(stringBuilder.length(), idx);
    stringBuilder.append(delim);

    StringBuilder output = new StringBuilder(word).reverse();
    stringBuilder.append(output);
}
// did we have trailing delimiter ?
if(fileContent.length()!=stringBuilder.length())
{ //append remaining
    stringBuilder.append(fileContent.substring(stringBuilder.length()));
}
return stringBuilder.toString();

Upvotes: 1

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627056

You may match and reverse only chunks of 1+ letters (with a simple \p{L}+ pattern) and keep the rest as is:

String s = "Joe'); MAKE TEST random;--";
StringBuffer result = new StringBuffer();
Matcher m = Pattern.compile("\\p{L}+").matcher(s);
while (m.find()) {
    String replacement = new StringBuilder(m.group()).reverse().toString();
    m.appendReplacement(result, replacement);
}
m.appendTail(result);
System.out.println(result.toString()); // => eoJ'); EKAM TSET modnar;--

See the Java demo online.

Upvotes: 1

Related Questions