Issue #9680 has been reported by Sam Rawlins.

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Bug #9680: String#sub and siblings should not use regex when String pattern is passed
https://bugs.ruby-lang.org/issues/9680

* Author: Sam Rawlins
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: trunk
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
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Currently `String#sub`, `#sub!`, `#gsub, and `#gsub!` all accept a String pattern, but immediately create a Regexp from it, and use the regex engine to search for the pattern. This is not performant. For example, `"123:456".gsub(":", "_")` creates the following objects, most of which are immediately up for GC:

* dup of the original String
* result String
* 2x `":"<US-ASCII>`
* 2x `":"<ASCII-8BIT>`
* Regexp from pattern: `/:/`
* `#<MatchData ":">`
* `#<MatchData nil>`

I have a solution which is not too complicated, at https://github.com/ruby/ruby/pull/579 and attached. Calls to `rb_reg_search()` are replaced with calls to a new function, `rb_pat_search()`, which conditionally calls `rb_reg_search()` or `rb_str_index()`, depending on whether the pattern is a String. Calculating the substring that needs to be replaced is also different when the pattern is a String.

Runtime of each method is dramatically reduced:

    require 'benchmark'
    
    n = 4_000_000
    Benchmark.bm(7) do |bm|
      str1 = "123:456"; str2 = "123_456";
      colon = ":"; underscore = "_"
      # each benchmark runs the substring method twice so that the bang methods can
      # perform the same number of substitutions to str1 each go around.
      bm.report("sub")   { n.times { str1.sub(colon, underscore);   str2.sub(underscore, colon) } }
      bm.report("sub!")  { n.times { str1.sub!(colon, underscore);  str1.sub!(underscore, colon) } }
      bm.report("gsub")  { n.times { str1.gsub(colon, underscore);  str2.gsub(underscore, colon) } }
      bm.report("gsub!") { n.times { str1.gsub!(colon, underscore); str1.gsub!(underscore, colon) } }
    end

    # trunk
                  user     system      total        real
    sub      40.450000   0.580000  41.030000 ( 41.209658)
    sub!     39.780000   0.580000  40.360000 ( 40.656789)
    gsub     58.500000   0.820000  59.320000 ( 59.603923)
    gsub!    59.400000   0.770000  60.170000 ( 60.435687)
    
    # this patch
                  user     system      total        real
    sub       3.060000   0.010000   3.070000 (  3.091920)
    sub!      2.380000   0.010000   2.390000 (  2.390769)
    gsub      7.130000   0.130000   7.260000 (  7.299139)
    gsub!     7.660000   0.150000   7.810000 (  7.846190)

When using a String pattern, runtime is reduced by 87% to 94%.

There is only one incompatibility that I am aware of: `$&` will not be set after using a sub method with a String pattern. (Subgroups (`$1`, ...) will not be available either, but weren't before, since String patterns are escaped before being used.)

In the future, only 3 more methods use the function, `get_pat()`, that creates a Regexp from the String pattern: `#split`, `#scan`, and `#match`. I think this fix could be applied to these as well.

---Files--------------------------------
ruby-579.diff (5.12 KB)


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