[ruby-core:54269] [ruby-trunk - Feature #8110] Regex methods not changing global variables

Issue #8110 has been updated by naruse (Yui NARUSE).


You may misunderstand, unlike Perl, Ruby's setting global variable cost is small.
Ruby only set a MatchData object to its scope.
$~ (Regexp.last_match) gets it.
The implementation of $& (Regexp.last_match[0]), $` (Regexp.last_match.pre_match), and $' (Regexp.last_match.post_match)
are get $~ and call [0], pre_match, or post_match.
So setting cost is very small (0.2 second for 1,000,000 times).

And if it doesn't set global variable, it means that it can't recycle previous MatchData object.
So it allocates new MatchData object each time, it costs both allocation and GC.
On following case, its cost is beyond the setting cost.
  r = Regexp.new(foo, Regexp::SKIP_GLOBALS); 1000000.times{r=~"foo"}

Therefore if you want speed up, you must remove making MatchData object.
String#match won't speed up so much because its API need creating MatchData object.
(moreover its current implementation uses $~)
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Feature #8110: Regex methods not changing global variables
https://bugs.ruby-lang.org/issues/8110#change-38555

Author: prijutme4ty (Ilya Vorontsov)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: next minor


It is useful to have methods allowing pattern matching without setting global variables. It can be very hard to understand where the problem is when you for example insert a string like `puts pat === my_str` and your program fails in a place which is far-far away from inserted place. This can happen due to replacing global variables of previous pattern match. I caught to this when placed pattern-match inside case-statement and shadowed global vars which were initially filled by match in when-statement.
For now one can extract pattern matching into another method thus defining method-scope for that variables. But sometimes it looks like an overkill. May be simple method like #match_globalsafe can prevent that kind of errors. At least when a programmer see such a method in a list of methods, he's warned that usual match can cause such problems.


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