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This is an excerpt from the latest version perlfaq4.pod, which
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4.21: How do I remove consecutive pairs of characters?

(contributed by brian d foy)

You can use the substitution operator to find pairs of characters (or
runs of characters) and replace them with a single instance. In this
substitution, we find a character in "(.)". The memory parentheses store
the matched character in the back-reference "\1" and we use that to
require that the same thing immediately follow it. We replace that part
of the string with the character in $1.


We can also use the transliteration operator, "tr///". In this example,
the search list side of our "tr///" contains nothing, but the "c" option
complements that so it contains everything. The replacement list also
contains nothing, so the transliteration is almost a no-op since it
won't do any replacements (or more exactly, replace the character with
itself). However, the "s" option squashes duplicated and consecutive
characters in the string so a character does not show up next to itself

my $str = 'Haarlem'; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York


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