Mirror Onigmo quirk for escaped number chains ...

Any escape of a number above 7 is treated as a literal escape (i.e. a redundant escape) IFF it is followed by another number.

```ruby
/\99/ # matches "99"
/9\9/ # SyntaxError
```

This is because Onigmo enters an octal-parsing branch for `\\\d{2,}` and then bails if any number is > 7.

fixes #87

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- handle a corner case where parsing redundant number escapes raised an error
+  * e.g. `parse(/\99/)`, which in Ruby is a valid Regexp that matches `99`
+  * thanks to [Markus Schirp](https://github.com/mbj) for the report
+
 ## [2.8.1] - 2023-06-10 - Janosch Müller
 
 ### Fixed

lib/regexp_parser/scanner/scanner.rl

@@ -267,6 +267,13 @@
       fret;
     };
 
+    [8-9] . [0-9] { # special case, emits two tokens
+      text = copy(data, ts-1, te)
+      emit(:escape, :literal, text[0, 2])
+      emit(:literal, :literal, text[2])
+      fret;
+    };
+
     meta_char {
       case text = copy(data, ts-1, te)
       when '\.';  emit(:escape, :dot,               text)

spec/parser/refcalls_spec.rb

@@ -77,6 +77,9 @@
 
   specify('parse invalid reference') do
     expect { RP.parse('\1') }.to raise_error(/Invalid reference/)
+    expect { RP.parse('1\1') }.to raise_error(/Invalid reference/)
+    expect { RP.parse('\8') }.to raise_error(/Invalid reference/)
+    expect { RP.parse('8\8') }.to raise_error(/Invalid reference/)
     expect { RP.parse('(a)\2') }.to raise_error(/Invalid reference/)
     expect { RP.parse('\k<1>') }.to raise_error(/Invalid reference/)
     expect { RP.parse('\k<+1>') }.to raise_error(/Invalid reference/)

spec/scanner/escapes_spec.rb

@@ -27,6 +27,10 @@
   include_examples 'scan', 'a\0124',          1 => [:escape,  :octal,            '\012',           1,  5]
   include_examples 'scan', '\712+7',          0 => [:escape,  :octal,            '\712',           0,  4]
 
+  # special case: "out-of-bound octal escapes" are not treated as backrefs
+  include_examples 'scan', '\80',             0 => [:escape,  :literal,          '\8',             0,  2]
+  include_examples 'scan', '\80',             1 => [:literal, :literal,          '0',              2,  3]
+
   include_examples 'scan', 'a\xA',            1 => [:escape,  :hex,              '\xA',            1,  4]
   include_examples 'scan', 'a\x24c',          1 => [:escape,  :hex,              '\x24',           1,  5]
   include_examples 'scan', 'a\x0640c',        1 => [:escape,  :hex,              '\x06',           1,  5]

spec/scanner/sets_spec.rb

@@ -40,6 +40,8 @@
   include_examples 'scan', '[\7]',                  1 => [:escape, :octal,           '\7',         1, 3]
   include_examples 'scan', '[\77]',                 1 => [:escape, :octal,           '\77',        1, 4]
   include_examples 'scan', '[\777]',                1 => [:escape, :octal,           '\777',       1, 5]
+  include_examples 'scan', '[\8]',                  1 => [:escape, :literal,         '\8',         1, 3]
+  include_examples 'scan', '[\88]',                 1 => [:escape, :literal,         '\8',         1, 3]
   include_examples 'scan', '[\\[]',                 1 => [:escape, :set_open,        '\[',         1, 3]
   include_examples 'scan', '[\\]]',                 1 => [:escape, :set_close,       '\]',         1, 3]
   include_examples 'scan', '[a\-]',                 2 => [:escape, :literal,         '\-',         2, 4]