CODEWITHSUNDEEP
javaregex
c12
c12

Reputation: 9827

Java RegEX To Split and Reverse String

I have a Java String "test/this/string" that I want to reverse to "string/this/test" using a regular expression or the most efficient Java algorithm. The way I know is to use the split method, loop over the array and rebuild the string manually. The number of "/" can vary and doesn't occur a fixed number of times. Any ideas?

Upvotes: 2

Views: 3380

Answers (3)

Bohemian
Bohemian

Reputation: 425003

This can be done in one line using regex:

String reversed = str.replaceAll("(.*?)(/.*?)?(/.*?)?(/.*?)?(/.*?)?(/.*?)?(/.*?)?(/.*?)?(/.*?)?", "$9$8$7$6$5$4$3$2$1").replaceAll("^/", "");;

The way this works is the regex matches the whole string and captures the parts as separate groups, then the replacement string has back referenced to those captured groups to put them back in the reverse order.

This works for 1-9 parts.

Upvotes: 1

Bernhard Barker
Bernhard Barker

Reputation: 55609

Here's my take:

String input = "test/this/string";
List<String> list = Arrays.asList(input.split("(?=/)|(?<=/)"));
Collections.reverse(list);
StringBuilder sb = new StringBuilder();
for (String s: list)
   sb.append(s);
System.out.println(sb.toString());

(?<=/) is a zero-length matching regex that matches if the previous character is a /.
(?=/) is a zero-length matching regex that matches if the next character is a /.

So (?=/)|(?<=/) matches right before and after each /, thus the split splits the string into "test", "/", "this", "/", "string".

Test.

Upvotes: 1

arshajii
arshajii

Reputation: 129507

Here's an idea, using lastIndexOf():

String str = "string/this/test";

StringBuilder rev = new StringBuilder(str.length());

int start = str.lastIndexOf('/'), end = str.length();

while (start >= 0) {
    rev.append(str.substring(start + 1, end));
    rev.append('/');

    end = start;
    start = str.lastIndexOf('/', start - 1);
}

rev.append(str.substring(0, end));

System.out.println(rev);

test/this/string

Upvotes: 1

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