Category: chapters

Use the /r substitution flag to work on a copy

How many times has this happened to you? You want to modify each element of an array so you send it through a map (Item 20. Use foreach, map, and grep as appropriate.). However, instead of your expected output, you only to get a bunch of numbers or empty strings back? For example, in this case, some digits got into the names of the cats and you want to remove them with a substitution: Continue reading “Use the /r substitution flag to work on a copy”

Understand “global” variables.

Perl has two sorts of variables: the lexical variables that are limited to a particular scope, and package variables that you define in a namespace. The package variables are sometimes also called global variables because they are visible from anywhere in the program as long as you know their name. As you might suspect, Perl makes it a bit more interesting: there are many sorts of global variables. Continue reading “Understand “global” variables.”

Use branch reset grouping to number captures in alternations

Perl’s regular expressions have a simple rule for capturing groups. It counts the order of left parentheses to assign capture variables. Not all capture groups must actually match parts of the string, and Perl doesn’t care if they do. Perl assigns capture groups inside an alternation consecutively, even though it knows that only one branch of the alternation will match. Perl 5.10 adds the branch reset, (?|alternation) which mitigates that, though. Continue reading “Use branch reset grouping to number captures in alternations”

Temporarily remove hash keys or array elements with `delete local`

Perl 5.12 adds a feature that lets you locally delete a hash key or array element (refresh your memory of local with Item 43: Know the difference between my and local. This new feature allows you to temporarily prune a hash or an array: Continue reading “Temporarily remove hash keys or array elements with `delete local`”

Turn off Perl 5.12 deprecation warnings, if you dare!

Perl 5.12 deprecates several features, for various reasons. Some of the features were always stupid, some need to make way for future development, and some are just too ornery to maintain. All of these are listed in the perldelta5120 documentation. The new thing, however, is that Perl 5.12 will warn you about these even if you don’t have warnings turned on. Consider this script full of Perl whoppers: Continue reading “Turn off Perl 5.12 deprecation warnings, if you dare!”

Match Unicode characters by property value

A Unicode character has properties; it knows things about itself. Perl v5.10 introduced a way to match a character that has certain properties that v5.10 supports. In some cases you can match a particular property value. Now v5.12 allows you can match any Unicode property by its value. The newly-supported ones include Numeric_Value and Age, for example:

\p{Numeric_Value: 1}
\p{Nv=7}
\p{Age: 3.0}

Continue reading “Match Unicode characters by property value”

Respect the global state of the flip flop operator

Perl’s flip-flop operator, .., (otherwise known as the range operator in scalar context) is a simple way to choose a window on some data. It returns false until its lefthand side is true. Once the lefthand side is true, the flip-flop operator returns true until its righthand side is true. Once the righthand side is true, the flip flop operator returns false. That is, the lefthand side turns it on and the righthand side turns it off. Continue reading “Respect the global state of the flip flop operator”

Detect regular expression match variables in your code

[UPDATE: this is not a problem in v5.18 and later.]

In Item 33: “Watch out for match variables”, you found out that the match variable $`, $&, and $` come with a performance hit. With all of the module code that you might use, you might be using those variables even though you didn’t code with them yourself. Continue reading “Detect regular expression match variables in your code”