diff --git a/lib/unit/system.rb b/lib/unit/system.rb
index 545ef84..c3fbfbf 100644
--- a/lib/unit/system.rb
+++ b/lib/unit/system.rb
@@ -47,6 +47,10 @@ def load(input)
 
       @unit.each {|name, unit| validate_unit(unit[:def]) }
 
+      # Newly registered symbols may introduce glyph characters the lexer must
+      # recognize; rebuild it lazily on next parse.
+      @tokenizer = nil
+
       true
     end
 
@@ -62,7 +66,7 @@ def validate_unit(units)
 
     def parse_unit(expr)
       stack, result, implicit_mul = [], [], false
-      expr.to_s.scan(TOKENIZER).each do |tok|
+      expr.to_s.scan(tokenizer).each do |tok|
         if tok == '('
           stack << '('
           implicit_mul = false
@@ -79,7 +83,7 @@ def parse_unit(expr)
           val = case tok
                 when REAL   then [[:one, tok.to_f, 1]]
                 when DEC    then [[:one, tok.to_i, 1]]
-                when SYMBOL then symbol_to_unit(tok)
+                else symbol_to_unit(tok)
                 end
           stack << '*' if implicit_mul
           implicit_mul = true
@@ -94,11 +98,28 @@ def parse_unit(expr)
 
     REAL   = /^-?(?:(?:\d*\.\d+|\d+\.\d*)(?:[eE][-+]?\d+)?|\d+[eE][-+]?\d+)$/
     DEC    = /^-?\d+$/
-    SYMBOL = /^[a-zA-Z_°'"][\w°'"]*$/
     OPERATOR = { '/' => ['/', 1], '*' => ['*', 1], '·' => ['*', 1], '^' => ['^', 2], '**' => ['^', 2] }
     OPERATOR_TOKENS = OPERATOR.keys.sort_by {|x| -x.size }. map {|x| Regexp.quote(x) }
-    VALUE_TOKENS = [REAL.source[1..-2], DEC.source[1..-2], SYMBOL.source[1..-2]]
-    TOKENIZER = Regexp.new((OPERATOR_TOKENS + VALUE_TOKENS + ['\\(', '\\)']).join('|'))
+    OPERATOR_CHARS = (OPERATOR.keys.join + '()').chars.uniq.freeze
+
+    # The tokenizer and symbol matcher are derived from the symbols actually
+    # registered in this system (plus an ASCII baseline), then memoized.
+    # #load clears the memo, so any unit defined later — including ones whose
+    # symbol uses a non-ASCII glyph (Ω, ℃, µ, %) — becomes lexable without
+    # touching the lexer. This replaces a frozen glyph char-class that silently
+    # dropped unrecognized glyphs to a dimensionless value.
+    def tokenizer
+      @tokenizer ||= Regexp.new(
+        (OPERATOR_TOKENS + [REAL.source[1..-2], DEC.source[1..-2], symbol_pattern, '\\(', '\\)']).join('|')
+      )
+    end
+
+    def symbol_pattern
+      glyphs = (unit_symbol.keys + factor_symbol.keys).join.chars.uniq.reject do |char|
+        char =~ /[A-Za-z0-9_]/ || char =~ /\s/ || OPERATOR_CHARS.include?(char)
+      end
+      "[A-Za-z0-9_#{glyphs.map { |char| Regexp.escape(char) }.join}]+"
+    end
 
     def lookup_symbol(symbol)
       if unit_symbol[symbol]
diff --git a/spec/system_spec.rb b/spec/system_spec.rb
index 8b87463..4b24eea 100644
--- a/spec/system_spec.rb
+++ b/spec/system_spec.rb
@@ -95,4 +95,36 @@
       end
     end
   end
+
+  describe "lexing every symbol in the default system" do
+    # Guards against the lexer silently dropping a registered glyph to a
+    # dimensionless value (the historical Ω/% failure): every unit symbol the
+    # system knows must parse back to that same unit. Built on a fresh system
+    # with the documented default load-out so the set is deterministic and not
+    # polluted by other specs loading optional systems onto Unit.default_system.
+    default = Unit::System.new("default") do |sys|
+      sys.load(:si)
+      sys.load(:binary)
+      sys.load(:degree)
+      sys.load(:time)
+    end
+    default.unit_symbol.each do |sym, name|
+      it "parses #{sym.inspect} as #{name}" do
+        expect(Unit(1, sym, default).unit).to eq(Unit(1, name.to_s, default).unit)
+      end
+    end
+  end
+
+  describe "lexing a glyph symbol loaded at runtime" do
+    # The lexer is derived from the registered symbols, so a unit defined after
+    # the system was built (e.g. an app loading a domain unit) becomes lexable
+    # without any change to the gem.
+    it "parses a non-word glyph symbol once its unit is loaded" do
+      system.load(:si)
+      system.load('units' => { 'percent' => { 'sym' => '%', 'def' => '1 / 100' } })
+
+      expect(Unit(1, '%', system).unit).to eq(Unit(1, 'percent', system).unit)
+      expect(Unit(50, '%', system)).to eq(Unit(Rational(1, 2), system))
+    end
+  end
 end