Lexr

Lexr is a lightweight lexical analyser written in ruby, it has no dependencies, has good test coverage, looks pretty and reads well.

An example: Expressions

ExpressionLexer = Lexr.that {
    ignores /\s+/ => :whitespace
    matches /[-+]?[0-9]*\.?[0-9]+/ => :number, :convert_with => lambda { |v| Float(v) }
    matches "+" => :addition
    matches "-" => :subtraction
    matches "*" => :multiplication
    matches "/" => :division
    matches "(" => :left_parenthesis
    matches ")" => :right_parenthesis
}

lexer = ExpressionLexer.new("1 * 12.5 / (55 + 2 - 56)")

while (token = lexer.next) != Lexr::Token.end
    puts token
end

results in an output of

number(1.0)
multiplication(*)
number(12.5)
division(/)
left_parenthesis(()
number(55.0)
addition(+)
number(2.0)
subtraction(-)
number(56.0)
right_parenthesis())

if you added a % in there somewhere, you'd get a Lexr::UnmatchableTextError with a message like this:

=> Unexpected character '%' at position 5

and that pretty is every feature so far. Please let me know of any bugs or additions that you'd like to see!