Regex negative matching with python

Question

I have a string which is allowed to contain only alphabets, numbers, empty spaces and symbol ':'. So I wrote the following code :

regex = r'![a-zA-Z0-9\:\ ]+'
print re.match(regex, myString)

However it does not seem to work. I tried different combinations r'?!([a-zA-Z0-9\:\ ])+' and also with re.search, but it does not seem to work. Any help?

Gergo Erdosi · Accepted Answer · 2013-11-18 15:37:05Z

2

If you want a positive match (valid characters), then use:

r'^[a-zA-Z0-9: ]+$'

If you want a negative match (whether the string has invalid characters), then:

r'[^a-zA-Z0-9: ]+'

edited Nov 18, 2013 at 15:37

answered Nov 18, 2013 at 15:12

Gergo Erdosi

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4 Comments

ridgerunner Over a year ago

For the first expression to work, you need to add anchors like so: r'^[a-zA-Z0-9: ]+$'. The second expression should work fine as-is.

avi Over a year ago

I am using negative matching. Its not not working, for both[valid and invalid] strings it re.match is returning None

Gergo Erdosi Over a year ago

It must be a Python thing. According to the documentation, match matches "zero or more characters at the beginning of string". Try search instead of match, so print re.search(regex, myString). See documentation: docs.python.org/2/library/re.html

avi Over a year ago

+1 and accepted. Worked with search, but I was doing a mistake. I was reading file line by line and I forgot to consider newline character. Now it is working. Thank you!

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