Make whitespace filters Unicode-aware

Previously we were only leveraging Ruby's `String#strip` to handle the
logic in these filters but that only covers ASCII whitespace. When
rendering Liquid templates into HTML it would be confusing for these
filters to not strip *all* whitespace.

Additionally, it's helpful when trying to compare two values in, say, a
Liquid conditional.

lib/liquid/standardfilters.rb

@@ -36,6 +36,19 @@ module Liquid
       %r{<style.*?</style>}m,
     )
     STRIP_HTML_TAGS = /<.*?>/m
+    # Use POSIX whitespace matching so filters handle whitespace beyond Ruby String#strip's ASCII set.
+    WHITESPACE_LEFT = /\A[[:space:]]+/
+    WHITESPACE_RIGHT = /[[:space:]]+\z/
+    WHITESPACE_EDGES = Regexp.union(WHITESPACE_LEFT, WHITESPACE_RIGHT)
+    # Optimized runs regex to find 2 or more [[:space:]] OR a single [[:space:]]
+    # that isn't already `" " `.
+    WHITESPACE_RUNS = /([[:space:]]{2,}|[[[:space:]]&&[^ ]])/
+    private_constant(
+      :WHITESPACE_EDGES,
+      :WHITESPACE_LEFT,
+      :WHITESPACE_RIGHT,
+      :WHITESPACE_RUNS,
+    )
 
     class << self
       def try_coerce_encoding(input, encoding:)
@@ -312,7 +325,7 @@ module Liquid
     def squish(input)
       return if input.nil?
 
-      Utils.to_s(input).strip.gsub(/\s+/, ' ')
+      Utils.to_s(input).gsub(WHITESPACE_RUNS, ' ').strip
     end
 
     # @liquid_public_docs
@@ -324,7 +337,7 @@ module Liquid
     # @liquid_return [string]
     def strip(input)
       input = Utils.to_s(input)
-      input.strip
+      input.gsub(WHITESPACE_EDGES, ' ').strip
     end
 
     # @liquid_public_docs
@@ -336,7 +349,7 @@ module Liquid
     # @liquid_return [string]
     def lstrip(input)
       input = Utils.to_s(input)
-      input.lstrip
+      input.gsub(WHITESPACE_LEFT, ' ').lstrip
     end
 
     # @liquid_public_docs
@@ -348,7 +361,7 @@ module Liquid
     # @liquid_return [string]
     def rstrip(input)
       input = Utils.to_s(input)
-      input.rstrip
+      input.gsub(WHITESPACE_RIGHT, ' ').rstrip
     end
 
     # @liquid_public_docs

test/integration/standard_filter_test.rb

@@ -169,6 +169,15 @@ class StandardFiltersTest < Minitest::Test
 \t   boo   " | squish }})).render)
     assert_equal("", Liquid::Template.parse('{{ nil | squish }}').render)
     assert_equal("", Liquid::Template.parse('{{ " " | squish }}').render)
+
+    unicode_spaces = "\u00A0\u202F\u2009\u2007"
+
+    assert_template_result(
+      "foo bar boo",
+      "{{ source | squish }}",
+      { 'source' => "#{unicode_spaces}foo\u202F\u2009bar\t\n\u2007boo#{unicode_spaces}" },
+    )
+    assert_template_result("\u200Bfoo\u200B", "{{ source | squish }}", { 'source' => "\u200Bfoo\u200B" })
   end
 
   def test_escape
@@ -703,16 +712,42 @@ class StandardFiltersTest < Minitest::Test
   def test_strip
     assert_template_result('ab c', "{{ source | strip }}", { 'source' => " ab c  " })
     assert_template_result('ab c', "{{ source | strip }}", { 'source' => " \tab c  \n \t" })
+
+    unicode_spaces = "\u00A0\u202F\u2009\u2007"
+
+    assert_template_result(
+      'ab c',
+      "{{ source | strip }}",
+      { 'source' => "#{unicode_spaces}ab c#{unicode_spaces}" },
+    )
+    assert_template_result("a\u00A0b\u202Fc", "{{ source | strip }}", { 'source' => "a\u00A0b\u202Fc" })
+    assert_template_result("\u200Bfoo\u200B", "{{ source | strip }}", { 'source' => "\u200Bfoo\u200B" })
   end
 
   def test_lstrip
     assert_template_result('ab c  ', "{{ source | lstrip }}", { 'source' => " ab c  " })
     assert_template_result("ab c  \n \t", "{{ source | lstrip }}", { 'source' => " \tab c  \n \t" })
+
+    unicode_spaces = "\u00A0\u202F\u2009\u2007"
+
+    assert_template_result(
+      "ab c#{unicode_spaces}",
+      "{{ source | lstrip }}",
+      { 'source' => "#{unicode_spaces}ab c#{unicode_spaces}" },
+    )
   end
 
   def test_rstrip
     assert_template_result(" ab c", "{{ source | rstrip }}", { 'source' => " ab c  " })
     assert_template_result(" \tab c", "{{ source | rstrip }}", { 'source' => " \tab c  \n \t" })
+
+    unicode_spaces = "\u00A0\u202F\u2009\u2007"
+
+    assert_template_result(
+      "#{unicode_spaces}ab c",
+      "{{ source | rstrip }}",
+      { 'source' => "#{unicode_spaces}ab c#{unicode_spaces}" },
+    )
   end
 
   def test_strip_newlines