Python RegEx - Negative Lookahead not working after a ? quantifier

I'm new to regex, and I'm wanting to find all instances of "po" and it's variants (i.e. "p.o. | p. o. | p o") that ISN'T followed by "box" because I'm interested in purchase orders and not PO boxes. The code below isn't working and just matches the po even when it's followed by a "box." Any ideas?

string = " po  pobox  po box  po  box    p.o.  p.o.box  p.o. box  p.o.  box"



re.findall(r' p.?s?o.?(?!s*box)', string)



//expected output

[' po', ' p.o.']



//actual output

[' po', ' p.o.', ' p.o', ' p.o', ' p.o']

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

1

The p.o in p.o.box is not followed by box. It's followed by ..

– user2357112
Nov 20 '18 at 22:36

Are you referring to the expected output part? It was suppose to read “p.o.” instead of “p.o” so box WOULD follow “p.o.”

– Will Blanton
Nov 21 '18 at 2:40

add a comment |

string = " po  pobox  po box  po  box    p.o.  p.o.box  p.o. box  p.o.  box"



re.findall(r' p.?s?o.?(?!s*box)', string)



//expected output

[' po', ' p.o.']



//actual output

[' po', ' p.o.', ' p.o', ' p.o', ' p.o']

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

1

The p.o in p.o.box is not followed by box. It's followed by ..

– user2357112
Nov 20 '18 at 22:36

Are you referring to the expected output part? It was suppose to read “p.o.” instead of “p.o” so box WOULD follow “p.o.”

– Will Blanton
Nov 21 '18 at 2:40

add a comment |

string = " po  pobox  po box  po  box    p.o.  p.o.box  p.o. box  p.o.  box"



re.findall(r' p.?s?o.?(?!s*box)', string)



//expected output

[' po', ' p.o.']



//actual output

[' po', ' p.o.', ' p.o', ' p.o', ' p.o']

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

string = " po  pobox  po box  po  box    p.o.  p.o.box  p.o. box  p.o.  box"



re.findall(r' p.?s?o.?(?!s*box)', string)



//expected output

[' po', ' p.o.']



//actual output

[' po', ' p.o.', ' p.o', ' p.o', ' p.o']

python regex

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

edited Nov 21 '18 at 2:38

asked Nov 20 '18 at 22:31

Will Blanton

234

asked Nov 20 '18 at 22:31

Will Blanton

234

asked Nov 20 '18 at 22:31

Will Blanton

234

1

The p.o in p.o.box is not followed by box. It's followed by ..

– user2357112
Nov 20 '18 at 22:36

Are you referring to the expected output part? It was suppose to read “p.o.” instead of “p.o” so box WOULD follow “p.o.”

– Will Blanton
Nov 21 '18 at 2:40

add a comment |

1

The p.o in p.o.box is not followed by box. It's followed by ..

– user2357112
Nov 20 '18 at 22:36

Are you referring to the expected output part? It was suppose to read “p.o.” instead of “p.o” so box WOULD follow “p.o.”

– Will Blanton
Nov 21 '18 at 2:40

The p.o in p.o.box is not followed by box. It's followed by ..

– user2357112
Nov 20 '18 at 22:36

Are you referring to the expected output part? It was suppose to read “p.o.” instead of “p.o” so box WOULD follow “p.o.”

– Will Blanton
Nov 21 '18 at 2:40

add a comment |

1 Answer
1

active

oldest

votes

You placed the lookahead after an optional pattern and backtracking makes it possible to match the string in another way.

If Python supported possessive quantifiers, it would be easy to solve by adding + after the .? that is before the lookahead: p.?s?o.?+(?!s*box). It would prevent the engine from backtracking into .? pattern.

However, since Python re does not support them, you need to move the lookahead right after the o, obligatory part, and add .? to the lookahead:

r'p.?s?o(?!.?s*box).?'

          ^^^^^^^^^^^^^

See the regex demo. Add b after box if you plan to match it as a whole word. Same with the first p, you may want to add a b before it to match p as a whole word.

Details

p - a p

.? - an optional (1 or 0) dots

s? - an optional (1 or 0) whitespaces

o - an o

(?!.?s*box) - a negative lookahead that fails the match if, immediately to the right of the current location there is an optional dot, 0+ whitespaces and box

.? - an optional (1 or 0) dots

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

1

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

add a comment |

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1 Answer
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1 Answer
1

active

oldest

votes

You placed the lookahead after an optional pattern and backtracking makes it possible to match the string in another way.

However, since Python re does not support them, you need to move the lookahead right after the o, obligatory part, and add .? to the lookahead:

r'p.?s?o(?!.?s*box).?'

          ^^^^^^^^^^^^^

See the regex demo. Add b after box if you plan to match it as a whole word. Same with the first p, you may want to add a b before it to match p as a whole word.

Details

p - a p

.? - an optional (1 or 0) dots

s? - an optional (1 or 0) whitespaces

o - an o

(?!.?s*box) - a negative lookahead that fails the match if, immediately to the right of the current location there is an optional dot, 0+ whitespaces and box

.? - an optional (1 or 0) dots

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

1

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

add a comment |

You placed the lookahead after an optional pattern and backtracking makes it possible to match the string in another way.

However, since Python re does not support them, you need to move the lookahead right after the o, obligatory part, and add .? to the lookahead:

r'p.?s?o(?!.?s*box).?'

          ^^^^^^^^^^^^^

See the regex demo. Add b after box if you plan to match it as a whole word. Same with the first p, you may want to add a b before it to match p as a whole word.

Details

p - a p

.? - an optional (1 or 0) dots

s? - an optional (1 or 0) whitespaces

o - an o

(?!.?s*box) - a negative lookahead that fails the match if, immediately to the right of the current location there is an optional dot, 0+ whitespaces and box

.? - an optional (1 or 0) dots

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

1

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

add a comment |

You placed the lookahead after an optional pattern and backtracking makes it possible to match the string in another way.

However, since Python re does not support them, you need to move the lookahead right after the o, obligatory part, and add .? to the lookahead:

r'p.?s?o(?!.?s*box).?'

          ^^^^^^^^^^^^^

See the regex demo. Add b after box if you plan to match it as a whole word. Same with the first p, you may want to add a b before it to match p as a whole word.

Details

p - a p

.? - an optional (1 or 0) dots

s? - an optional (1 or 0) whitespaces

o - an o

(?!.?s*box) - a negative lookahead that fails the match if, immediately to the right of the current location there is an optional dot, 0+ whitespaces and box

.? - an optional (1 or 0) dots

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

You placed the lookahead after an optional pattern and backtracking makes it possible to match the string in another way.

However, since Python re does not support them, you need to move the lookahead right after the o, obligatory part, and add .? to the lookahead:

r'p.?s?o(?!.?s*box).?'

          ^^^^^^^^^^^^^

See the regex demo. Add b after box if you plan to match it as a whole word. Same with the first p, you may want to add a b before it to match p as a whole word.

Details

p - a p

.? - an optional (1 or 0) dots

s? - an optional (1 or 0) whitespaces

o - an o

(?!.?s*box) - a negative lookahead that fails the match if, immediately to the right of the current location there is an optional dot, 0+ whitespaces and box

.? - an optional (1 or 0) dots

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

edited Nov 20 '18 at 22:39

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

answered Nov 20 '18 at 22:34

Wiktor Stribiżew

313k16133210

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

1

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

add a comment |

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

1

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

That’s exactly what the issue was! I thought my overalll expression was right, it was just that backtracking issue causing the false positives. But now, the last period is chopped off. Is there a way to get it to still show if it’s there?

– Will Blanton
Nov 21 '18 at 2:37

Nevermind! I missed the “.?” at the end. That took care of that! Thanks again!

– Will Blanton
Nov 21 '18 at 2:56

add a comment |

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