正则表达式查找完全匹配

Question

I have the following scenarios:

message: ON
message: "ON"
message: 'ON'
message: "TONC"
message: Los Angeles, ON
message: HOLDS ON FILE

I want to match the word ON in the the first three scenarios, and not the others.

I tried with positive/negative lookaheads/behinds to no avail. ex:

\bON\b(?=[\s{0,1}\"{0,1}\'{0,1}]$)

Answer 1

I recommend munging the string to eliminate the stuff you don't want rather than trying to match directly. Here I'm just removing non-alpha characters and comparing against the desired result:

#!/usr/bin/env python3
import re

tests = ['message: ON',
'message: "ON"',
'message: \'ON\'',
'message: "TONC"',
'message: Los Angeles, ON',
'message: HOLDS ON FILE']

for test in tests:
  test = re.sub(r'\W+', '', test)
  print(test)
  print(test == "messageON")

Answer 2

You may use

r'''(?<=:\s)(["']?)\bON\b\1'''

See the regex demo . If necessary, add $ at the end of the pattern to make it match at the end of the string.

Details

• (?<=:\s) - a positive lookbehind that matches a location that is immediately preceded with : and a whitespace char
• (["']?) - Group 1: an optional " or ' char
• \bON\b - whole word ON
• \1 - Backreference to Group 1 value, either " or ' .

NOTE : If there can be a variable amount of whitespaces between : and the ON word, you may use PyPi regex module and use r'''(?<=:\s*)(["']?)\bON\b\1''' pattern.

Answer 3

Why not

message: (['"])?ON(?(1)\1)

See a demo on regex101.com .

Answer 4

If you only want to match only ON , another option is to use an alternation matching either between a colon followed by a space and a whitespace boundary at the right or between the same type of quotes.

(?<=: )ON(?!\S)|(?<=: (['"]))ON(?=\1)

• (?<=: ) Asserting : and space on the left
• ON Match ON
• (?!\S) Assert a whitespace boundary on the right
• | Or
• (?<=: (['"])) Assert : and space on the left and capture in group 1 ' or "
• ON Match ON
• (?=\1) Assert on the right the same quote that is matched in group 1 using a backreference

Regex demo

Answer 5

I basically just add message: in the regex ( r'^message: ["\']?\bON\b["\']?$' ):

#!/usr/bin/env python3

from re import search

strings = [
'message: ON',
'message: "ON"',
'message: \'ON\'',
'message: "TONC"',
'message: Los Angeles, ON',
'message: HOLDS ON FILE',
]

for s in strings:
    m = search(r'^message: ["\']?\bON\b["\']?$', s)
    if m is not None:
        print(s)
        print("It's a match !")

and it works, here is the output:

$ ./regex_to_find_exact_match.py
message: ON
It's a match !
message: "ON"
It's a match !
message: 'ON'
It's a match !