Transliteration - Page 16

While we're looking at regular expressions, we should briefly consider another operator. While it's not directly associated with regexps, the transliteration operator has a lot in common with them and adds a very useful facility to the matching and substitution techniques we've already seen.

What this does is to correlate the characters in its two arguments, one by one, and use these pairings to substitute individual characters in the referenced string. It uses the syntax tr/one/two/ and (as with the matching and substitution operators) references the special variable $_ unless otherwise specified with =~ or !~. In this case, it replaces all the 'o's in the referenced string with 't's, all the 'n's with 'w's, and all the 'e's with 'o's.

Let's say you wanted to replace, for some reason, all the numbers in a string with letters. You might say something like this:

$string =~ tr/0123456789/abcdefghij/;

This would turn, say, "2011064" into "cabbage". You can use ranges in transliteration but not in any of the character classes. We could write the above as:

$string =~ tr/0-9/a-j/;

The return value of this operator is, by default, the number of characters matched with those in the first argument. You can therefore use the transliteration operator to count the number of occurrences of certain characters. For example, to count the number of vowels in a string, you can use:

my $vowels = $string =~ tr/aeiou//;

Note that this will not actually substitute any of the vowels in the variable $string. As the second argument is blank, there is no correlation, so no substitution occurs. However, the transliteration operator can take the /d modifier, which will delete occurrences on the left that do not have a correlating character on the right. So, to get rid of all spaces in a string quickly, you could use this line:

$string =~ tr/ //d;

Common Blunders

There are a few common mistakes people tend to make when writing regexps. We've already seen that /a*b*c*/ will happily match any string at all, since it matches each letter zero times. What else can go wrong?

Forgetting To Group: /Bam{2}/ will match 'Bamm', while /(Bam){2}/ will match 'BamBam', so be careful when choosing which one to use. The same goes for alternation: /Simple|on/ will match 'Simple' and 'on', while /Sim(ple|on)/ will match both 'Simple' and 'Simon' Group each option separately.

Getting The Anchors Wrong: ^ goes at the beginning, $ goes at the end. A dollar anywhere else in the string makes perl try and interpolate a variable.

Forgetting To Escape Special Characters: Do you want them to have a special meaning? These are the characters to be careful of: . * ? + [ ] ( ) { } ^ $ | and of course \ itself.

Not Counting from Zero: The first entry in an array is given the index zero.

Counting from Zero: I know, I know! All along I've been telling you that computers start counting from zero. Nevertheless, there's always the odd exception - the first backreference is $1. Don't blame Perl though - it took this behavior from a language called awk which used $1 as the first reference variable.

Split - Page 15
Beginning Perl
More Advanced Topics - Page 17