369/8 Extracting Domains from URLs - what's wrong with my Regex here in using Lookbehind

[willx]

Hi i know my question is similar to threads

369-8 Extracting Domains from URLs

Hey everyone, I tried to solve a problem from the Advanced Regular Expressions. The problem is from the step 8. I’ve written down my code. But the inspector says that the variable, domains, is wrong desoite the fact that the top_domains I wrote is correct. I do not know why I am wrong. So I need some help. The following is my code. test_urls = pd.Series([ ‘https://www.amazon.com/Technology-Ventures-Enterprise-Thomas-Byers/dp/0073523429’, ‘http://www.interactivedynamicvideo.com/’, ‘http…

Errors in Advance Regular Expression Exercise 8 - Extracting Domains from URLs

@vincentmkh Sorry for the late response! It’s possible that there might be a domain name inside domains that isn’t caught by the test_urls_clean variable. As for copy-pasting, remember that you can make code completely readable and copy-pastable by enveloping them in 3 back ticks (```). So ```abc```, if enclosed in 3 back-ticks, will appear as a coded block: abc In this case I already edited your earlier post and did that for you so I could copy-paste your exact code. You can check the formatt…

i tried this

(?<=//)[\w+.]+

in https://regexr.com which seems to work fine in extracting

‘https://www.amazon.com/Technology-Ventures-Enterprise-Thomas-Byers/dp/0073523429’,
‘http://www.interactivedynamicvideo.com/’,
‘http://www.nytimes.com/2007/11/07/movies/07stein.html?_r=0’,
‘http://evonomics.com/advertising-cannot-maintain-internet-heres-solution/’,
‘HTTPS://github.com/keppel/pinn’,
‘Http://phys.org/news/2015-09-scale-solar-youve.html’,
‘https://iot.seeed.cc’,
‘http://www.bfilipek.com/2016/04/custom-deleters-for-c-smart-pointers.html’,
‘http://beta.crowdfireapp.com/?beta=agnipath’,
‘https://www.valid.ly?param’

however, when i run it in Jupyter notebook, i get

---

ValueError Traceback (most recent call last)
in
----> 1 test_urls_clean = test_urls.str.extract(pattern_url, expand=False)

~/anaconda/lib/python3.6/site-packages/pandas/core/strings.py in extract(self, pat, flags, expand)
2765 @copy(str_extract)
2766 def extract(self, pat, flags=0, expand=True):
-> 2767 return str_extract(self, pat, flags=flags, expand=expand)
2768
2769 @copy(str_extractall)

~/anaconda/lib/python3.6/site-packages/pandas/core/strings.py in str_extract(arr, pat, flags, expand)
848 return _str_extract_frame(arr._orig, pat, flags=flags)
849 else:
–> 850 result, name = _str_extract_noexpand(arr._parent, pat, flags=flags)
851 return arr._wrap_result(result, name=name, expand=expand)
852

~/anaconda/lib/python3.6/site-packages/pandas/core/strings.py in _str_extract_noexpand(arr, pat, flags)
711
712 regex = re.compile(pat, flags=flags)
–> 713 groups_or_na = _groups_or_na_fun(regex)
714
715 if regex.groups == 1:

~/anaconda/lib/python3.6/site-packages/pandas/core/strings.py in _groups_or_na_fun(regex)
686 “”“Used in both extract_noexpand and extract_frame”""
687 if regex.groups == 0:
–> 688 raise ValueError(“pattern contains no capture groups”)
689 empty_row = [np.nan] * regex.groups
690

ValueError: pattern contains no capture groups

Can someone walk through with me what is wrong in my regex pattern here

[Prem]

Hi @willx

After seeing the error, according to me:

ValueError: Says that pattern does not contain any capture group and you are using extract method to match pattern(In first line of error).

pattern = r'https?://([\w\.]+)'  # pattern to extract domain from url.
Series.str.extract(pattern, flags=re.I)  # ignorecase flag to avoid Https, HTTPS.

Extract method will return the string, which match the capture group (expression inside of bracket) of pattern.

Here, Capture group is “[\w\.]+” .

Hope this help .

[willx]

Hi @Prem, i know what the answer is. what i am trying to find out is why, we can’t just capture at (?<=//) instead of having to look at https?://

[Prem]

(?<=abc)xyz --> Matches xyz only when it is preceded by ‘abc’.

Use pattern = r’((?<=//)[\w\.]+)’

On covering expression in brackets extract will understand, extract [\w\.]+ only when it is preceded by // .

[willx]

yes i know that because i tried using that on https://regexr.com and it worked there, but it doesn’t work on Jupyter notebook. that’s what i am trying to find out where the differences is

[Prem]

Hey @willx

I tried on my local computer Jupyter Notebook, it worked perfectly. You can see the below image.

[willx]

hi so sorry, was taking a hiatus for the past 2 weeks. Happy New year by the way

yeah, it is a mistake on my part, i missed out the parentheses in my earlier

i did this

(?<=//)[\w+.]+

what you have shown me is that i missed out here

pattern = r"((?<=//)[\w+.]+)"

thanks it works now. boy what a bummer. thank you so much

[Prem]

It happens sometimes. And according to Indian standard time, we have 4 hours for the new year, but for you Happy New Year In Advance .