How to extract numbers from a string in Python?

Question or problem about Python programming:

I would extract all the numbers contained in a string. Which is the better suited for the purpose, regular expressions or the isdigit() method?

Example:

line = "hello 12 hi 89"

Result:

[12, 89]

How to solve the problem:

Solution 1:

If you only want to extract only positive integers, try the following:

>>> str = "h3110 23 cat 444.4 rabbit 11 2 dog"
>>> [int(s) for s in str.split() if s.isdigit()]
[23, 11, 2]

I would argue that this is better than the regex example because you don’t need another module and it’s more readable because you don’t need to parse (and learn) the regex mini-language.

This will not recognize floats, negative integers, or integers in hexadecimal format. If you can’t accept these limitations, jmnas’s answer below will do the trick.

Solution 2:

I’d use a regexp :

>>> import re
>>> re.findall(r'\d+', 'hello 42 I\'m a 32 string 30')
['42', '32', '30']

This would also match 42 from bla42bla. If you only want numbers delimited by word boundaries (space, period, comma), you can use \b :

>>> re.findall(r'\b\d+\b', 'he33llo 42 I\'m a 32 string 30')
['42', '32', '30']

To end up with a list of numbers instead of a list of strings:

>>> [int(s) for s in re.findall(r'\b\d+\b', 'he33llo 42 I\'m a 32 string 30')]
[42, 32, 30]

Solution 3:

This is more than a bit late, but you can extend the regex expression to account for scientific notation too.

import re

# Format is [(, ), ...]
ss = [("apple-12.34 ba33na fanc-14.23e-2yapple+45e5+67.56E+3",
       ['-12.34', '33', '-14.23e-2', '+45e5', '+67.56E+3']),
      ('hello X42 I\'m a Y-32.35 string Z30',
       ['42', '-32.35', '30']),
      ('he33llo 42 I\'m a 32 string -30', 
       ['33', '42', '32', '-30']),
      ('h3110 23 cat 444.4 rabbit 11 2 dog', 
       ['3110', '23', '444.4', '11', '2']),
      ('hello 12 hi 89', 
       ['12', '89']),
      ('4', 
       ['4']),
      ('I like 74,600 commas not,500', 
       ['74,600', '500']),
      ('I like bad math 1+2=.001', 
       ['1', '+2', '.001'])]

for s, r in ss:
    rr = re.findall("[-+]?[.]?[\d]+(?:,\d\d\d)*[\.]?\d*(?:[eE][-+]?\d+)?", s)
    if rr == r:
        print('GOOD')
    else:
        print('WRONG', rr, 'should be', r)

Gives all good!

Additionally, you can look at the AWS Glue built-in regex

Solution 4:

I’m assuming you want floats not just integers so I’d do something like this:

l = []
for t in s.split():
    try:
        l.append(float(t))
    except ValueError:
        pass

Note that some of the other solutions posted here don’t work with negative numbers:

>>> re.findall(r'\b\d+\b', 'he33llo 42 I\'m a 32 string -30')
['42', '32', '30']

>>> '-3'.isdigit()
False

Solution 5:

If you know it will be only one number in the string, i.e ‘hello 12 hi’, you can try filter.

For example:

In [1]: int(''.join(filter(str.isdigit, '200 grams')))
Out[1]: 200
In [2]: int(''.join(filter(str.isdigit, 'Counters: 55')))
Out[2]: 55
In [3]: int(''.join(filter(str.isdigit, 'more than 23 times')))
Out[3]: 23

But be carefull !!! :

In [4]: int(''.join(filter(str.isdigit, '200 grams 5')))
Out[4]: 2005

Hope this helps!