Programming Perl, 4th Edition

The tr/// Operator (Transliteration)

tr/SEARCHLIST/REPLACEMENTLIST/cdsr

 LVALUE =~ tr/SEARCHLIST/REPLACEMENTLIST/cds
 RVALUE =~ tr/SEARCHLIST/REPLACEMENTLIST/cdsr
 RVALUE =~ tr/SEARCHLIST//c

For sed devotees, y/// is provided as a synonym for tr///. This is why you can’t call a function named y, any more than you can call a function named q or m. In all other respects, y/// is identical to tr///, and we won’t mention it again.

This operator might not appear to fit into a chapter on pattern matching since it doesn’t use patterns. This operator scans a string, character by character, and replaces each occurrence of a character found in SEARCHLIST (which is not a regular expression) with the corresponding character from REPLACEMENTLIST (which is not a replacement string). It looks a bit like m// and s///, though, and you can even use the =~ or !~ binding operators on it, so we describe it here. (qr// and split are pattern-matching operators, but you don’t use the binding operators on them, so they’re elsewhere in the book. Go figure.)

Transliteration returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is altered. The SEARCHLIST and REPLACEMENTLIST may define ranges of sequential characters with a dash:

$message =~ tr/A–Za–z/N–ZA–Mn–za–m/;    # rot13 encryption.

Note that a range like A–Z assumes a linear character set like ASCII. But each character set has its own ideas of how characters are ordered and thus of which characters ...