Effective Perl Programming – write better, more idiomatic Perl
The qr// operator allows you to compile a regular expression without applying it to anything. You get the pattern without the match, and you can reuse the pattern as often as you like. Before v5.30, it had an inconsistency with \N{} sequences, but that’s fixed now.
Continue reading “Perl 5.30 fixes single quoted qr” with \N{}”
Before Perl v5.10 introduced state variables, people did various things to create persistent lexical variables for a subroutine. With v5.30, one of those constructs is now a fatal error.
Often you want a persistent variable to be scoped and private to a subroutine. But, once you leave that scope, normal lexical variables disappear because their reference count drops to zero. So, no persistence.
Continue reading “No more false postfix lexical declarations in v5.30”
Earlier this year, this website was the target of some sort of attack in which a bot sent seemingly random data in its requests. The attack wasn’t that big of a deal since I easily blocked it with Cloudflare, but it was interesting. The apparently random data was actually a mix of Latin, Hangul, and Cyrillic. Domain hacks with unusual Unicode characters shows some of these exploits. Curiously, v5.28 added some regex feature that deals with this sort of nonsense.
You can sometimes improve the performance of your regular expression by preventing parts of it from backtracking when you know that might be useful. Item 38. Avoid unnecessary backtracking had many techniques for this, although it did not mention atomic matching (a feature added in v5.005).
Continue reading “Use atomic matching for complex non-backtracking”
[This feature stabilizes in Perl v5.32]
Perl v5.28 adds more-readable, alternate spelled-out forms for some of its regular expression extended patterns. Then, to make those slightly less readable, there are very short initialisms for those. Although these might seem superfluous now, the ability to define new syntax without relying on the limited number of ASCII symbols.
Continue reading “Use alpha assertions for more understandable regexes”
Perl v5.29 is the development series leading up to the maintenance release v5.30 sometime in the middle of 2019. As it’s released—roughly monthly—you can get a peek at what’s coming up. You can track the progress by reading the perldelta documentation that comes with each Perl release (although you’ll need to select the development version you want to inspect).
qr'\N{name}' is now supported
Does the {N,} really match infinite repetitions in a Perl regular expression? No, it never has. You’ve been limited to 32,766 repetitions. Perl v5.30 is about to double that for you. And, if you are one of the people who needed more, I’d like to hear your story.
Continue reading “Perl v5.30 lets you match more with the general quantifier”
Perl v5.26 adds three new special variables related to captures. The @{^CAPTURE} is an array of all the capture buffers. %{^CAPTURE} is a alias for %+ and stores the actually-matched named capture labels as its keys. %{^CAPTURE_ALL} is an alias for %- and stores all the named capture labels and their matched (or not) values.
Continue reading “Use @{^CAPTURE} to get a list of all the capture buffers”
When you store a large string in a scalar, perl allocates the memory to store that string and associate it with the scalar. It uses the same memory even if you assign a much shorter value to the same scalar. Use the functional form of undef to let perl reuse that memory for something else. This is important when you want to reuse the variable or the lifetime of the variable is very long.
Perl v5.26 adds the experimental declared_refs feature that expands on the experimental refaliasing feature from v5.22. As with all experimental features, this may change or disappear according to perlpolicy.
Continue reading “Mix assignment and reference aliasing with declared_refs”