The parse function takes in a string. space, tabs, and newlines are ignored.

soumyeahdood.rkt

#lang racket
(require parser-tools/lex
         (prefix-in : parser-tools/lex-sre))

(define-lex-trans ::? 
  (lambda (stx)
    (syntax-case stx ()
      [(_ v b)
       #'(:: v (:? " ") b)])))

(define progl
  (lexer
   [(eof) #f]
   [(:or #\space #\tab) (progl input-port)]
   [(::? "so" "dood") ">"]
   [(::? "yeah" "dood") "<"]
   [(::? "so" "um") "+"]
   [(::? "yeah" "um") "-"]
   [(::? "so" "...") "."]
   [(::? "yeah" "...") ","]
   [(::? "so" "?") "["]
   [(::? "yeah" "?") "]"]))


(define (parse s)
  (define o (open-input-string s))
   (let loop ()
     (define t (progl o))
     (if (not t)
         ""
         (string-append t (loop)))))