Extracting number from string in Python with regex

Question

I want to extract and print a variable number '-34.99' from the string:

myString = "Test1 [cm]:     -35.00/-34.99/-34.00/0.09"

The values in the string will change. How can I do it with the regular expression in Python?

Thanks in advance

Answer 1

Non-regex solution is:

myString = "Test1 [cm]:     -35.00/-34.99/-34.00/0.09"  
print myString.split("/")[1]

Test this code here.

---

One of regex solutions would be:

import re 
myString = "Test1 [cm]:     -35.00/-34.99/-34.00/0.09" 
print re.search(r'(?<=\/)[+-]?\d+(?:\.\d+)?', myString).group()

Test this code here.

Explanation:

(?<=\/)[+-]?\d+(?:\.\d+)?
└──┬──┘└─┬─┘└┬┘└───┬────┘
   │     │   │     │
   │     │   │     └ optional period with one or more trailing digits
   │     │   │
   │     │   └ one or more digits
   │     │
   │     └ optional + or -
   │
   └ leading slash before match 

Answer 2

For something like this, re.findall works great:

>>> import re
>>> myString = "Test1 [cm]:     -35.00/-34.99/-34.00/0.09"
>>> re.findall(r'([+-]?\d+\.\d+)',myString)
['-35.00', '-34.99', '-34.00', '0.09']

You can get the floats directly with a list comprehension:

>>> [float(f) for f in re.findall(r'([+-]?\d+\.\d+)',myString)]
[-35.0, -34.99, -34.0, 0.09]

Or the second one like this:

>>> re.findall(r'([+-]?\d+\.\d+)',myString)[1]
'-34.99'

The question will be how big a range of textual floating points will you accept? Some with no decimal points? Exponents?

>>> myString = "Test1 [cm]:     -35.00/-34.99/-34.00/0.09/5/1.0e6/1e-6"  

Ouch! -- this is getting harder with a regex.

You actually may be better off just using Python's string ops:

>>> ''.join([s for s in myString.split() if '/' in s]).split('/')
['-35.00', '-34.99', '-34.00', '0.09', '5', '1.0e6', '1e-6']

You can get the nth one same way:

>>> n=2
>>> ''.join([s for s in myString.split() if '/' in s]).split('/')[n]
'-34.00'

Then all the weird cases work without a harder regex:

>>> map(float,''.join([s for s in myString.split() if '/' in s]).split('/'))
[-35.0, -34.99, -34.0, 0.09, 5.0, 1000000.0, 1e-06]