Is there a regular expression that return substrings from a string, that do not match a given list of specific substrings?

Question

Hi I am wondering if there is a regular expression that can do the following:

Select all the substrings from a string that :

• start with & and
• have n number of characters after the & (n >= 0)

AND those substrings are NOT

• &
• '
• <
• > or
• "

For example, given the string

'Stewie & Brian    &partners in crime;'

is there a regex that will return only the substring &partners ?

My intuition says no , because I need a context free grammar but how can I prove that? Is there a regex to test it with the pumping lemma ?

Or a regex actually exists and my intuition is just wrong?

Thank you

Answer 1

Sure:

&(?!(amp|apos|lt|gt);)\S{4,}

for n=4

See live demo.

The key here is the negative look ahead (?!(amp|apos|lt|gt);), which asserts (without consuming input) that the input immediately following does not match (amp|apos|lt|gt);