import re

myText = 'brb some sample text I lov u. I need some $$ for 2mw.'

dictionary = {
  'brb': 'be right back',
  'lov u': 'love you',
  '$$': 'money',
  '2mw': 'tomorrow'
}

pattern = re.compile(r'(?<!\w)(' + '|'.join(re.escape(key) for key in dictionary.keys()) + r')(?!\w)')
result = pattern.sub(lambda x: dictionary[x.group()], myText)

print(result)