Use \A instead of ^

^ matches the beginning of every line. Given the string
function() {
}
using /^\{/ will match the final } because it's at the beginning of the
line

lib/ahem/lexer.rb

     @position += 1 while !at_end && @source.slice(@position).match(/\s/)
 
     source = @source.slice(@position..-1)
-    if source.match(/^null/)
+
+    if source.match(/\Anull/)
       @position += 4
       Token.new(TokenKinds::NULL)
-    elsif source.match(/^true/)
+    elsif source.match(/\Atrue/)
       @position += 4
       Token.new(TokenKinds::BOOLEAN, true)
-    elsif source.match(/^false/)
+    elsif source.match(/\Afalse/)
       @position += 5
       Token.new(TokenKinds::BOOLEAN, false)
-    elsif source.match(/^if/)
+    elsif source.match(/\Aif/)
       @position += 2
       Token.new(TokenKinds::IF)
-    elsif source.match(/^elseif/)
+    elsif source.match(/\Aelseif/)
       @position += 6
       Token.new(TokenKinds::ELSEIF)
-    elsif source.match(/^else/)
+    elsif source.match(/\Aelse/)
       @position += 4
       Token.new(TokenKinds::ELSE)
-    elsif source.match(/^\d+(\.\d+)?/)
-      number = source.match(/^\d+(\.\d+)?/)[0]
+    elsif source.match(/\A\d+(\.\d+)?/)
+      number = source.match(/\A\d+(\.\d+)?/)[0]
       @position += number.size
       Token.new(TokenKinds::NUMBER, number.to_f)
     elsif source[0] == '"'
       end
 
       Token.new(TokenKinds::STRING, string)
-    elsif source.match(/^\:([a-z][a-zA-Z0-9_]*)/)
-      atom = source.match(/^\:([a-z][a-zA-Z0-9_]*)/)[1]
+    elsif source.match(/\A\:([a-z][a-zA-Z0-9_]*)/)
+      atom = source.match(/\A\:([a-z][a-zA-Z0-9_]*)/)[1]
       @position += atom.size + 1
       Token.new(TokenKinds::ATOM, atom.to_sym)
-    elsif source.match(/^\+/)
+    elsif source.match(/\A\+/)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :+)
-    elsif source.match(/^\-/)
+    elsif source.match(/\A\-/)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :-)
-    elsif source.match(/^\*/)
+    elsif source.match(/\A\*/)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :*)
-    elsif source.match(%r{^\/})
+    elsif source.match(%r{\A\/})
       @position += 1
       Token.new(TokenKinds::OPERATOR, :/)
-    elsif source.match(/^\{/)
+    elsif source.match(/\A\{/)
       @position += 1
       Token.new(TokenKinds::LBRACE)
-    elsif source.match(/^\}/)
+    elsif source.match(/\A\}/)
       @position += 1
       Token.new(TokenKinds::RBRACE)
-    elsif source.match(/^\(/)
+    elsif source.match(/\A\(/)
       @position += 1
       Token.new(TokenKinds::LPAREN)
-    elsif source.match(/^\)/)
+    elsif source.match(/\A\)/)
       @position += 1
       Token.new(TokenKinds::RPAREN)
-    elsif source.match(/^\[/)
+    elsif source.match(/\A\[/)
       @position += 1
       Token.new(TokenKinds::LBRACKET)
-    elsif source.match(/^\]/)
+    elsif source.match(/\A\]/)
       @position += 1
       Token.new(TokenKinds::RBRACKET)
-    elsif source.match(/^\;/)
+    elsif source.match(/\A\;/)
       @position += 1
       Token.new(TokenKinds::SEMICOLON)
-    elsif source.match(/^,/)
+    elsif source.match(/\A,/)
       @position += 1
       Token.new(TokenKinds::COMMA)
-    elsif source.match(/^\./)
+    elsif source.match(/\A\./)
       @position += 1
       Token.new(TokenKinds::DOT)
-    elsif source.match(/^=>/)
+    elsif source.match(/\A=>/)
       @position += 2
       Token.new(TokenKinds::ROCKET)
-    elsif source.match(/^!/)
+    elsif source.match(/\A!/)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :!)
-    elsif source.match(/^==/)
+    elsif source.match(/\A==/)
       @position += 2
       Token.new(TokenKinds::OPERATOR, :==)
-    elsif source.match(/^\<=/)
+    elsif source.match(/\A\<=/)
       @position += 2
       Token.new(TokenKinds::OPERATOR, :<=)
-    elsif source.match(/^\>\=/)
+    elsif source.match(/\A\>\=/)
       @position += 2
       Token.new(TokenKinds::OPERATOR, :>=)
-    elsif source.match(/^\</)
+    elsif source.match(/\A\</)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :<)
-    elsif source.match(/^\>/)
+    elsif source.match(/\A\>/)
       @position += 1
       Token.new(TokenKinds::OPERATOR, :>)
-    elsif source.match(/^and/)
+    elsif source.match(/\Aand/)
       @position += 3
       Token.new(TokenKinds::OPERATOR, :and)
-    elsif source.match(/^or/)
+    elsif source.match(/\Aor/)
       @position += 2
       Token.new(TokenKinds::OPERATOR, :or)
-    elsif source.match(/^not/)
+    elsif source.match(/\Anot/)
       @position += 3
       Token.new(TokenKinds::OPERATOR, :not)
-    elsif source.match(/^\=/)
+    elsif source.match(/\A\=/)
       @position += 1
       Token.new(TokenKinds::EQUALS)
-    elsif source.match(/^let/)
+    elsif source.match(/\Alet/)
       @position += 3
       Token.new(TokenKinds::LET)
-    elsif source.match(/^function/)
+    elsif source.match(/\Afunction/)
       @position += 8
       Token.new(TokenKinds::FUNCTION)
-    elsif source.match(/^class/)
+    elsif source.match(/\Aclass/)
       @position += 5
       Token.new(TokenKinds::CLASS)
-    elsif source.match(/^public/)
+    elsif source.match(/\Apublic/)
       @position += 6
       Token.new(TokenKinds::PUBLIC)
-    elsif source.match(/^private/)
+    elsif source.match(/\Aprivate/)
       @position += 7
       Token.new(TokenKinds::PRIVATE)
-    elsif source.match(/^for/)
+    elsif source.match(/\Afor/)
       @position += 3
       Token.new(TokenKinds::FOR)
-    elsif source.match(/^in/)
+    elsif source.match(/\Ain/)
       @position += 2
       Token.new(TokenKinds::IN)
-    elsif source.match(/^[a-z][a-zA-Z0-9_]*/)
-      identifier = source.match(/^[a-z][a-zA-Z0-9_]*/)[0]
+    elsif source.match(/\A[a-z][a-zA-Z0-9_]*/)
+      identifier = source.match(/\A[a-z][a-zA-Z0-9_]*/)[0]
       @position += identifier.size
       Token.new(TokenKinds::IDENTIFIER, identifier)
-    elsif source.match(/^[A-Z][a-zA-Z0-9_]*/)
-      class_name = source.match(/^[A-Z][a-zA-Z0-9_]*/)[0]
+    elsif source.match(/\A[A-Z][a-zA-Z0-9_]*/)
+      class_name = source.match(/\A[A-Z][a-zA-Z0-9_]*/)[0]
       @position += class_name.size
       Token.new(TokenKinds::CLASS_NAME, class_name)
     else