Basic Regex Check

regex_check.py

import re
text_list = [
    "P1:field1",
    "P1:field2"
    "P2:field1"
    "P2:field2",
    "P3:field1",
    "P3:field2",
    "P4:field1",
    "P5:field1",
    "p2:field1"
    "endingwith P1:",
    "endingwith P2:",
    "endingwith P3:"
    "other-field"
]

pattern = re.compile("[P|p][^1-2]:")
for text in text_list:
    if pattern.match(text):
        print(text)