Python regex for number with or without decimals using a dot or comma as separator?

Question

I'm just learning regex and now I'm trying to match a number which more or less represents this:

[zero or more numbers][possibly a dot or comma][zero or more numbers]

No dot or comma is also okay. So it should match the following:

1
123
123.
123.4
123.456
.456
123,  # From here it's the same but with commas instead of dot separators
123,4
123,456
,456

But it should not match the following:

0.,1
0a,1
0..1
1.1.2
100,000.99  # I know this and the one below are valid in many languages, but I simply want to reject these
100.000,99

So far I've come up with [0-9]*[.,][0-9]*, but it doesn't seem to work so well:

>>> import re
>>> r = re.compile("[0-9]*[.,][0-9]*")
>>> if r.match('0.1.'): print 'it matches!'
...
it matches!
>>> if r.match('0.abc'): print 'it matches!'
...
it matches!

I have the feeling I'm doing two things wrong: I don't use match correctly AND my regex is not correct. Could anybody enlighten me on what I'm doing wrong? All tips are welcome!

Answer 1

You need to make [.,] part as optional by adding ? after that character class and also don't forget to add anchors. ^ asserts that we are at the start and $ asserts that we are at the end.

^\d*[.,]?\d*$

DEMO

>>> import re
>>> r = re.compile(r"^\d*[.,]?\d*$")
>>> if r.match('0.1.'): print 'it matches!'
... 
>>> if r.match('0.abc'): print 'it matches!'
... 
>>> if r.match('0.'): print 'it matches!'
... 
it matches!

If you don't want to allow a single comma or dot then use a lookahead.

^(?=.*?\d)\d*[.,]?\d*$

DEMO

Answer 2

Your regex would work fine if you just add the ^ at the front and the $ at the back so that system knows how your string would begin and end.

Try this

^[0-9]*[.,]{0,1}[0-9]*$

import re

checklist = ['1', '123', '123.', '123.4', '123.456', '.456', '123,', '123,4', '123,456', ',456', '0.,1', '0a,1', '0..1', '1.1.2', '100,000.99', '100.000,99', '0.1.', '0.abc']

pat = re.compile(r'^[0-9]*[.,]{0,1}[0-9]*$')

for c in checklist:
   if pat.match(c):
      print '%s : it matches' % (c)
   else:
      print '%s : it does not match' % (c)

1 : it matches
123 : it matches
123. : it matches
123.4 : it matches
123.456 : it matches
.456 : it matches
123, : it matches
123,4 : it matches
123,456 : it matches
,456 : it matches
0.,1 : it does not match
0a,1 : it does not match
0..1 : it does not match
1.1.2 : it does not match
100,000.99 : it does not match
100.000,99 : it does not match
0.1. : it does not match
0.abc : it does not match