Python Regex: don't match if containing some string

Question

I'm using python regex on these two strings:

t1 = 'foo 36 months bar'

t2 = 'bar Not suitable for children under 36 months foo'

I want to match '36 months' on t1, and not match t2 because it has 'Not suitable for children under' before the '36 months.

After searching I have this:

regex = re.compile(r'((?!Not suitable for children under) 36 month)')

But it matches both. How do adapt this so it doesn't match a string with 'Not suitable for children under' before the ' 36 months'?

many thanks

mechanical_meat · Accepted Answer · 2020-02-10 00:54:57Z

2

What you're doing is a negative lookahead.
However, what you're trying to find is when some text exists before what you want to match.
So instead use a negative lookbehind:

re.search('(?<!Not suitable for children under) 36 months',t1)

Demonstration:

In [13]: re.search('(?<!Not suitable for children under) 36 months',t1)                             
Out[13]: <re.Match object; span=(3, 13), match=' 36 months'>

In [14]: re.search('(?<!Not suitable for children under) 36 months',t2)

answered Feb 10, 2020 at 0:54

mechanical_meat

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