Creating a single regex rather than a nested statement

Question

I want to do the following match:

match if MBzz is in the string but not if [Rr][Ee][Ff] is in the string

So the following should match:

klasdlkMBzzsdld
MBzz

And the following should not match:

RefmmmmMBzzmmmmm
MBzzmmmmmmREFmmmm

etc.

For now, I am doing this terrible hack:

def mySearch(val):
    if (re.compile('MBab').search(val) is not None) and \
       (re.compile('[Rr][Ee][Ff]').search(val) is None):
        return re.compile('MBab').search(val).group()
    return None

However, I feel that for something that is as simple as this, I should be able to accomplish this as a one liner.

For some reason, my computer did an autocorrect for regex to reggae — ssm
– ssm, Commented Jun 29, 2015 at 17:22

Kasravnd · Accepted Answer · 2015-06-29 17:53:03Z

2

You can use following regex with modifier i for ignoring the case :

^(?:(?!ref).)*(?=MBzz)(?:(?!ref).)*$

Demo

regex=re.compile(r'^[^ref]*(?=MBzz)[^ref]*$',re.I|re.MULTILINE)

The positive look behind (?=MBzz) will ensure your regex engine that your string contains MBzz and following negative look behind (?:(?!ref).)* will match any thing except ref.

And if you want yo consider the case for MBzz you can use following regex without ignore case modifier :

^(?:(?![rR][eE][fF]).)*(?=MBzz)(?:(?![rR][eE][fF]).)*$

edited Jun 29, 2015 at 17:53

answered Jun 29, 2015 at 17:24

Kasravnd

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1 Comment

ssm Over a year ago

Thanks for your answer. Its nearly exactly what I want, except that I do want to match the case for MBzz. But the positive and negative look behinds look very interesting. Ill definitely take a look at them.

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