Tag System - Up The Swanie

I usually seperate the functions into 3 sections. Input, lexical analyzer, and parser. The input just sends the next character to the lexical analyzer, the lexical analyzer strings the input into tokens, and the parser actually acts upon the whole thing.

Const TOKEN_EOL = 0
Const TOKEN_TAG = 1
Const TOKEN_STRING = 2

Global push = False
Global pushed$
Global message$
Global messagepointer = 1
Global token$
Global TextX = 0
Global TextY = 0


;character input
Function GetCharacter$()
If push = True ;if a character has been pushed onto the buffer, send it first
 DebugLog pushed$
 push = False
 Return pushed$
End If
If messagepointer > Len(message$) Then Return "eol"
c$ = Mid$(message$,messagepointer,1)
messagepointer = messagepointer + 1
Return c$
End Function

Function PushCharacter(c$)
Pushed = c$
Push = True
End Function

Function Lex()

c$ = GetCharacter()
Select c$
Case "["
 c$ = GetCharacter()
 Token = ""
 While c$ <> "]" And c$<> "eol"
  Token = Token + c$
  c$ = GetCharacter()
 Wend 
 Return TOKEN_TAG
Case "eol"
 Token$ = ""
 Return TOKEN_EOL
Default
 Token$ = ""
 While c$ <> "["And c$<>"eol"
  Token = Token + c$
  c$ = GetCharacter()
 Wend
 PushCharacter(c$)
 Return TOKEN_STRING
End Select
End Function

Function DisplayText(TextString$)
Message$ = TextString$
Repeat
 TokenValue = Lex()
 DebugLog TokenValue +" " + Token$
 Select TokenValue
 Case TOKEN_EOL
  Return
 Case TOKEN_TAG
  Select Lower(Token$)
  Case "r"
   Color 255,0,0
  Case "g"
   Color 0,255,0
  Case "b"
   Color 0,0,255
  Case "w"
   Color 255,255,255
  Case "p"
   Color 255,0,255
  Case "n"
   Color 212,212,212
  Case "return"
   TextX = 0
   TextY = TextY + 16
  Default
   RuntimeError "Don't recognize tag " + Token
  End Select
 Default
  Text TextX,TextY,Token$
  TextX = TextX + 8*Len(Token)
 End Select
Forever
End Function

Graphics 800,600
m$ = "[R]Please [G]Change [B]My [P]colors[Return]"
m$ = m$ + "[N]And this is default / normal text and this should be [B]blue Text"

DisplayText(m$)
WaitKey()

I know that seems like a lot for just changing colors, but it'll make it easier when you add functions, as well, as changing input. Like you could change to reading from a file or over a network connection just by changing the GetCharacter routine.