Programming language lexer FOX wrote a lexer for his programming language, what can be changed or fixed?

Question

Python

Good day, I wrote more than a normal lexer for my language in python.
This lexer is simple because at the initial stage I don’t want to write a mountain of code.
here is the code

#Лексер
import sys
import re

def lex(characters, token_exprs):
  pos = 0
  token = []
  while pos < len(characters):
    match = None
    for token_expr in token_exprs:
      pattern, tag = token_expr
      regex = re.compile(pattern)
      match = regex.match(characters, pos)
      if match:
        text = match.group(0)
        if tag:
          token = (text, tag)
          tokens.append(token)
        break
    if not match:
      sys.stderr.write('Illegal characters: %s\n' % characters[pos])
      sys.exit(1)
    else:
      pos = match.end(0)
  return tokens

  import lexer

  RESERVED = 'RESERVED'
  INT = 'INT'
  ID = 'ID'

  token_exprs = [
    (r'[ \n\t]+',              None),
    (r'#[^\n]*',               None),
    (r'\:=',                   RESERVED),
    (r'\(',                    RESERVED),
    (r'\)',                    RESERVED),
    (r';',                     RESERVED),
    (r'\+',                    RESERVED),
    (r'-',                     RESERVED),
    (r'\*',                    RESERVED),
    (r'/',                     RESERVED),
    (r'<=', RESERVED),
    (r'<', RESERVED),
    (r'>=', RESERVED),
    (r'>', RESERVED),
    (r'=',                     RESERVED),
    (r'!=',                    RESERVED),
    (r'and',                   RESERVED),
    (r'or',                    RESERVED),
    (r'not',                   RESERVED),
    (r'if',                    RESERVED),
    (r'then',                  RESERVED),
    (r'else',                  RESERVED),
    (r'while',                 RESERVED),
    (r'do',                    RESERVED),
    (r'end',                   RESERVED),
    (r'[0-9]+',                INT),
    (r'[A-Za-z][A-Za-z0-9_]*', ID),
]

def imp_lex(characters):
    return lexer.lex(characters, token_exprs)

#Присваиваем глобальную переменую для integer
x := 1

#Условие
if x = 1 then
   y := 2
else
  y := 3
end

#Цикл while
while x < 10 do
  x := x + 1
end

#Составные операторы(Раздельные)
x := 1;
y := 2

#Вычесление факториала
n := 5;
p := 1;
while n > 0 do
  p := p * n;
  n := n - 1
end

I would like to ask what can be added, what can be removed if possible, you can send the corrected code