Python Regex Cannot get expression to match

Question

I have a string from which I wish to extract a value. The string is: 'data - hk = "136 HK"'. I know, that data - hk = will always precede the value, hence I can use simple python str.split(). It is not a neat solution. I would like to get more familiar with regex, but my attempts have so far failed.

Here is what I have tried:

        text = 'data - hk = "136 HK"'
        
        # With simple text split - returns 136
        int(text.split("data - hk = ")[1].split('"')[1].split(" HK")[0])

        # With regex - returns nothing
        re.search("[\n\r].*data - hk:\s*([^\n\r]*)", str(text))

Can someone guide me to, what I need to change in the regex?

It could be something like re.findall(r'^data - hk = "(\d+) HK"', text). But would the captured substring always be an integer? Looks like it by your own attempt at split. — JvdV
– JvdV, Commented Apr 10, 2021 at 5:39
I forgot the end string anchor in my previous comment. @Rva92. Did it end up working for you? — JvdV
– JvdV, Commented Apr 10, 2021 at 5:52
yes it did, thank you very much. Post it as answer and I'll mark it — RVA92
– RVA92, Commented Apr 10, 2021 at 5:54

JvdV · Accepted Answer · 2021-04-10 06:18:29Z

2

It seems as though you can just validate your string and extract your integer using re.findall:

re.findall(r'^data - hk = "(\d+) HK"$', text)

See an online demo

^ - Start line anchor.
data - hk = " - A literal match for mentioned string.
(\d+) - A capture group to retrieve the integer of 1+ digits.
HK" - Another literal match for mentioned string.
$ - End line anchor.

edited Apr 10, 2021 at 6:18

answered Apr 10, 2021 at 6:01

JvdV

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Ryszard Czech · Accepted Answer · 2021-04-10 21:47:37Z

Use

data - hk\D*(\d+(?:\.\d+)?)

See proof.

EXPLANATION

--------------------------------------------------------------------------------
  data - hk                'data - hk'
--------------------------------------------------------------------------------
  \D*                      non-digits (all but 0-9) (0 or more times
                           (matching the most amount possible))
--------------------------------------------------------------------------------
  (                        group and capture to \1:
--------------------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
--------------------------------------------------------------------------------
    (?:                      group, but do not capture (optional
                             (matching the most amount possible)):
--------------------------------------------------------------------------------
      \.                       '.'
--------------------------------------------------------------------------------
      \d+                      digits (0-9) (1 or more times
                               (matching the most amount possible))
--------------------------------------------------------------------------------
    )?                       end of grouping
--------------------------------------------------------------------------------
  )                        end of \1

Code:

import re
s = 'data - hk = "136 HK"'
match = re.search(r'data - hk\D*(\d+(?:\.\d+)?)', s)
if match is not None:
    print(match.group(1))

Results: 136.

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