How to Match Strings Not Starting With Pattern using Regex and Python

John D K -

Need to match strings which don't start with a given pattern using a regular expression in Python?

If so, you may use the following syntax to find all strings except ones which don't start with https:

r"^(?!https).*"

Step 1: Match strings not starting with pattern

IN this example we have a list of URLs. Let's say that you would like to get all of them which don't start with https.

For this purpose we will use negative lookahead:

import re
texts = [
'https://en.wikipedia.org/wiki/Main_Page/',
'http://en.wikipedia.org/wiki/National_Park_Service/',
'https://en.wikipedia.org/wiki/Hoover_Dam/',
'http://en.wikipedia.org/wiki/United_States_Bureau_of_Reclamation/',
'https://en.wikipedia.org/wiki/Central_African_Republic/',
'en.wikipedia.org/wiki/Africa/',
'ftp://en.wikipedia.org/wiki/Central_African_Republic/',
]
for text in texts:
    print(re.findall(r"^(?!https).*", text))

The result is:

[]
['http://en.wikipedia.org/wiki/National_Park_Service/']
[]
['http://en.wikipedia.org/wiki/United_States_Bureau_of_Reclamation/']
[]
['en.wikipedia.org/wiki/Africa/']
['ftp://en.wikipedia.org/wiki/Central_African_Republic/']

How does it work?

  • ^ - asserts position at start of the string
  • (?!https) - Negative Lookahead - assert that the regex does not match - https
  • .* - matches any character between zero and unlimited times

Step 2: Match strings not starting with several patterns

Now let try to find all strings which don't start with:

  • https
  • ftp

We can use | which is or in regex syntax - r"^(?!https|ftp).*":

for text in texts:
    print(re.findall(r"^(?!https|ftp).*", text))

result is:

[]
['http://en.wikipedia.org/wiki/National_Park_Service/']
[]
['http://en.wikipedia.org/wiki/United_States_Bureau_of_Reclamation/']
[]
['en.wikipedia.org/wiki/Africa/']
[]

Note: you can add many patterns with |.

Step 3: Match strings not starting with list of characters

Finally let's see how to match all strings that don't start with several characters like:

  • f
  • h

This time we are going to list all characters between square brackets: [^hf]. Statement [hf] means match letters - f or h while ^ negates the match.

In other words match a single character not present in the list - [hf].

So we can use:

for text in texts:
    print(re.findall(r"^[^hf].*", text))

Which will give us:

[]
[]
[]
[]
[]
['en.wikipedia.org/wiki/Africa/']
[]