Positive Lookahead

x(?=y) matches with x only if it is followed by y

import re 
re.findall(r"\b[a-z]+(?=\d+\b)", "abc69 def ghi4 jk20l xyz")
# ['abc', 'ghi']

Negative Lookahead

x(?!y) matches with x only if it is NOT followed by y

import re 
re.findall(r"\b[a-z]+(?!\d+\b)", "abc69 def ghi4 jk20l xyz")
# ['ab', 'def', 'gh', 'jk', 'xyz']

(?<=x)y matches with y only if it preceded by x

import re 
re.findall(r"(?<=[ck])\d+", "abc69 def ghi4 jk20l xyz")
# ['69', '20']

(?<!x)y matches with y only if it is NOT preceded by x

import re 
re.findall(r"(?<![a-z]{3})\d+", "abc69 def ghi4 jk20l xyz")
# ['9', '20']