Reduced regular expression processing in the form of processing white space first (#237)

## Benchmark
```
RUBYLIB= BUNDLER_ORIG_RUBYLIB= /Users/naitoh/.rbenv/versions/3.4.1/bin/ruby -v -S benchmark-driver /Users/naitoh/ghq/github.com/naitoh/rexml/benchmark/parse.yaml
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
Calculating -------------------------------------
                         before       after  before(YJIT)  after(YJIT)
                 dom     19.849      20.109        36.064       38.655 i/s -     100.000 times in 5.038102s 4.972864s 2.772838s 2.586981s
                 sax     30.339      30.449        52.946       54.873 i/s -     100.000 times in 3.296102s 3.284176s 1.888722s 1.822391s
                pull     34.785      34.916        65.808       65.219 i/s -     100.000 times in 2.874810s 2.863976s 1.519581s 1.533305s
              stream     34.766      34.921        61.920       63.277 i/s -     100.000 times in 2.876359s 2.863571s 1.615000s 1.580354s

Comparison:
                              dom
         after(YJIT):        38.7 i/s
        before(YJIT):        36.1 i/s - 1.07x  slower
               after:        20.1 i/s - 1.92x  slower
              before:        19.8 i/s - 1.95x  slower

                              sax
         after(YJIT):        54.9 i/s
        before(YJIT):        52.9 i/s - 1.04x  slower
               after:        30.4 i/s - 1.80x  slower
              before:        30.3 i/s - 1.81x  slower

                             pull
        before(YJIT):        65.8 i/s
         after(YJIT):        65.2 i/s - 1.01x  slower
               after:        34.9 i/s - 1.88x  slower
              before:        34.8 i/s - 1.89x  slower

                           stream
         after(YJIT):        63.3 i/s
        before(YJIT):        61.9 i/s - 1.02x  slower
               after:        34.9 i/s - 1.81x  slower
              before:        34.8 i/s - 1.82x  slower
```
- YJIT=ON : 0.99x - 1.07x faster
- YJIT=OFF : 1.00x - 1.01x faster

Author: naitoh

lib/rexml/parsers/baseparser.rb

@@ -297,10 +297,11 @@ def pull_event
                 raise REXML::ParseException.new(message, @source)
               end
               name = parse_name(base_error_message)
-              if @source.match?(/\s*\[/um, true)
+              @source.match?(/\s*/um, true) # skip spaces
+              if @source.match?("[", true)
                 id = [nil, nil, nil]
                 @document_status = :in_doctype
-              elsif @source.match?(/\s*>/um, true)
+              elsif @source.match?(">", true)
                 id = [nil, nil, nil]
                 @document_status = :after_doctype
                 @source.ensure_buffer
@@ -312,9 +313,10 @@ def pull_event
                   # For backward compatibility
                   id[1], id[2] = id[2], nil
                 end
-                if @source.match?(/\s*\[/um, true)
+                @source.match?(/\s*/um, true) # skip spaces
+                if @source.match?("[", true)
                   @document_status = :in_doctype
-                elsif @source.match?(/\s*>/um, true)
+                elsif @source.match?(">", true)
                   @document_status = :after_doctype
                   @source.ensure_buffer
                 else
@@ -409,7 +411,8 @@ def pull_event
               id = parse_id(base_error_message,
                             accept_external_id: true,
                             accept_public_id: true)
-              unless @source.match?(/\s*>/um, true)
+              @source.match?(/\s*/um, true) # skip spaces
+              unless @source.match?(">", true)
                 message = "#{base_error_message}: garbage before end >"
                 raise REXML::ParseException.new(message, @source)
               end

test/parse/test_document_type_declaration.rb

@@ -153,7 +153,7 @@ def test_no_literal
 Line: 3
 Position: 26
 Last 80 unconsumed characters:
- SYSTEM>  <r/> 
+SYSTEM>  <r/> 
           DETAIL
         end
 
@@ -200,7 +200,7 @@ def test_content_double_quote
 Line: 3
 Position: 62
 Last 80 unconsumed characters:
- PUBLIC 'double quote " is invalid' "r.dtd">  <r/> 
+PUBLIC 'double quote " is invalid' "r.dtd">  <r/> 
             DETAIL
           end
 
@@ -228,10 +228,10 @@ def test_garbage_after_literal
             end
             assert_equal(<<-DETAIL.chomp, exception.to_s)
 Malformed DOCTYPE: garbage after external ID
-Line: 3
-Position: 65
+Line: 1
+Position: 58
 Last 80 unconsumed characters:
-x'>  <r/> 
+x'>
            DETAIL
           end