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Perl Cookbook by Nathan Torkington, Tom Christiansen

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Reversing a String by Word or Character

Problem

You want to reverse the characters or words of a string.

Solution

Use the reverse function in scalar context for flipping bytes.

$revbytes = reverse($string);

To flip words, use reverse in list context with split and join:

$revwords = join(" ", reverse split(" ", $string));

Discussion

The reverse function is two different functions in one. When called in scalar context, it joins together its arguments and returns that string in reverse order. When called in list context, it returns its arguments in the opposite order. When using reverse for its byte-flipping behavior, use scalar to force scalar context unless it’s entirely obvious.

$gnirts   = reverse($string);       # reverse letters in $string

@sdrow    = reverse(@words);        # reverse elements in @words

$confused = reverse(@words);        # reverse letters in join("", @words)

Here’s an example of reversing words in a string. Using a single space, " “, as the pattern to split is a special case. It causes split to use contiguous whitespace as the separator and also discard any leading null fields, just like awk. Normally, split discards only trailing null fields.

# reverse word order
$string = 'Yoda said, "can you see this?"';
@allwords    = split(" ", $string);
$revwords    = join(" ", reverse @allwords);
print $revwords, "\n";

                  this?" see you "can said, Yoda

We could remove the temporary array @allwords and do it on one line:

$revwords = join(" ", reverse split(" ", $string));

Multiple whitespace in $string becomes a single space in $revwords. If you want to preserve whitespace, use this:

$revwords = join("", reverse split(/(\s+)/, $string));

One use of reverse is to test whether a word is a palindrome (a word that reads the same backward or forward):

$word = "reviver";
$is_palindrome = ($word eq reverse($word));

We can turn this into a one-liner that finds big palindromes in /usr/dict/words.

% perl -nle 'print if $_ eq reverse && length > 5' /usr/dict/words
deedeed
               degged
               deified
               denned
               hallah
               kakkak
               murdrum
               redder
               repaper
               retter
               reviver
               rotator
               sooloos
               tebbet
               terret
               tut-tut

See Also

The split, reverse, and scalar functions in perlfunc (1) and Chapter 3 of Programming Perl ; the “Switches” section of perlrun (1) and Chapter 6 of Programming Perl

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