python regex match string does not start with

Question

I want to match any string that does not start with 4321 I came about it with the positive condition: match any string that starts with 4321:

^4321.*

regex here

Now I want to reverse that condition, for example:

1234555 passes
12322222 passess
None passess
4321ZZZ does not pass
43211111 does not pass

Please help me find the simplest regex as possible that accomplishes this.

I am using a mongo regex but the regex object is build in python so please no python code here (like startswith)

Idos · Accepted Answer · 2016-01-10 15:10:33Z

19

You could use a negative look-ahead (needs a multiline modifier):

^(?!4321).*

You can also use a negative look-behind (doesn't match empty string for now):

(^.{1,3}$|^.{4}(?<!4321).*)

Note: like another answer stated, regex is not required (but is given since this was the question verbatim) -> instead just use if not mystring.startswith('4321').

Edit: I see you are explicitly asking for a regex now so take my first one it's the shortest I could come up with ;)

edited Jan 10, 2016 at 15:10

answered Jan 10, 2016 at 14:42

Idos

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8 Comments

WebQube Over a year ago

regex101.com/r/fC1wX4/10 - am I missing something? I'm getting 0 matches even though I should get 2 out of 3 here

andreas-hofmann Over a year ago

It does match, if you add the multiline-modifier to the regex: regex101.com/r/fC1wX4/12

Idos Over a year ago

It works for me: regex101.com/r/qN5nM8/1 and regex101.com/r/hU1tO4/1

Casimir et Hippolyte Over a year ago

Why do you exclude empty strings?

Idos Over a year ago

@CasimiretHippolyte what exactly do you mean? I am matching only strings that don't start with 4321.

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TigerhawkT3 · Accepted Answer · 2016-01-10 14:55:01Z

7

You don't need a regex for that. Just use not and the startswith() method:

if not mystring.startswith('4321'):

You can even just slice it and compare equality:

if mystring[:4] != '4321':

answered Jan 10, 2016 at 14:55

TigerhawkT3

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Comments

Hidde · Accepted Answer · 2016-01-10 14:47:01Z

0

Why don't you match the string, and negate the boolean value using not:

import re
result = re.match('^4321.*', value)
if not result:
    print('no match!')

answered Jan 10, 2016 at 14:47

Hidde

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WebQube · Accepted Answer · 2016-01-11 07:44:30Z

0

Thank, @idos. For a complete answer I used the mongo's $or opertator

mongo_filter = {'$or': [{'db_field': re.compile("^(?!4321).*$")}, {'db_field1': {'$exists': False}}]})

This ensure not only strings that starts with 4321 but also if the field does not exists or is None

answered Jan 11, 2016 at 7:44

WebQube

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