Fix parsing ruleset shorthands in ruby 3.2

For reasons I've not been smart enough to figure out, there's a
difference in matching between 3.1 and 3.2

```ruby
RE_LENGTH_OR_PERCENTAGE =
Regexp.new('([\-]*(([0-9]*\.[0-9]+)|[0-9]+)(e[mx]+|px|[cm]+m|p[tc+]|in|\%))',
Regexp::IGNORECASE)
RE_BACKGROUND_POSITION = Regexp.new("((((#{RE_LENGTH_OR_PERCENTAGE})|left|center|right|top|bottom)[\s]*){1,2})", Regexp::IGNORECASE | Regexp::EXTENDED)
"  top right".match(RE_BACKGROUND_POSITION)
```

matches "top right" on ruby 3.1, but just "top " on 3.2.

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.7', '3.0', '3.1', 'jruby-head'] # TODO: fix '3.2',
+        ruby-version: ['2.7', '3.0', '3.1', '3.2', 'jruby-head']
 
     steps:
       - uses: actions/checkout@v2

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+ * Fix parsing background shorthands in ruby 3.2 [#140](https://github.com/premailer/css_parser/pull/140)
+
 ### Version v1.14.0
 
  * Fix parsing of multiline URL values for rule sets [#97](https://github.com/premailer/css_parser/pull/97)

lib/css_parser/regexps.rb

@@ -52,8 +52,10 @@ def self.regex_possible_values(*values)
   # Special units
   BOX_MODEL_UNITS_RX = /(auto|inherit|0|(-*([0-9]+|[0-9]*\.[0-9]+)(rem|vw|vh|vm|vmin|vmax|e[mx]+|px|[cm]+m|p[tc+]|in|%)))([\s;]|\Z)/imx.freeze
   RE_LENGTH_OR_PERCENTAGE = Regexp.new('([\-]*(([0-9]*\.[0-9]+)|[0-9]+)(e[mx]+|px|[cm]+m|p[tc+]|in|\%))', Regexp::IGNORECASE)
-  RE_BACKGROUND_POSITION = Regexp.new("((((#{RE_LENGTH_OR_PERCENTAGE})|left|center|right|top|bottom)[\s]*){1,2})", Regexp::IGNORECASE | Regexp::EXTENDED)
-  RE_BACKGROUND_SIZE = Regexp.new("\\s*/\\s*((((#{RE_LENGTH_OR_PERCENTAGE})|auto|cover|contain|initial|inherit)[\\s]*){1,2})", Regexp::IGNORECASE | Regexp::EXTENDED)
+  RE_SINGLE_BACKGROUND_POSITION = /#{RE_LENGTH_OR_PERCENTAGE}|left|center|right|top|bottom/i
+  RE_SINGLE_BACKGROUND_SIZE = /#{RE_LENGTH_OR_PERCENTAGE}|auto|cover|contain|initial|inherit/i
+  RE_BACKGROUND_POSITION = /#{RE_SINGLE_BACKGROUND_POSITION}\s+#{RE_SINGLE_BACKGROUND_POSITION}|#{RE_SINGLE_BACKGROUND_POSITION}/
+  RE_BACKGROUND_SIZE = %r{\s*/\s*(#{RE_SINGLE_BACKGROUND_SIZE}\s+#{RE_SINGLE_BACKGROUND_SIZE}|#{RE_SINGLE_BACKGROUND_SIZE})}
   FONT_UNITS_RX = /((x+-)*small|medium|larger*|auto|inherit|([0-9]+|[0-9]*\.[0-9]+)(e[mx]+|px|[cm]+m|p[tc+]|in|%)*)/i.freeze
   RE_BORDER_STYLE = /(\s*^)?(none|hidden|dotted|dashed|solid|double|dot-dash|dot-dot-dash|wave|groove|ridge|inset|outset)(\s*$)?/imx.freeze
   RE_BORDER_UNITS = Regexp.union(BOX_MODEL_UNITS_RX, /(thin|medium|thick)/i)