Question

I have a follow up question to this post: RegEx to exclude match if a certain word is present, but not another partial word .

If given the following string.

x*vec*grad(vec)

How do I find ‘vec’ but not ‘grad(vec)’?

I am assuming I will need to use something like the following for the search expression, but this doesn’t work for me.

vec(?!(?<=grad\(vec))

I also want to ignore the space inside the parentheses, i.e. it should work for the following.

x*vec*grad( vec )

I am using MATLAB’s regexp function. Thanks.

Answer 1

This works in c#, it should port to MATLAB easily enough:

(?<!grad\()vec

It matches vec*grad but not x*grad(vec).

The trick is to use the negative lookbehind (<!) instead of tyring to invert the positive lookbehind (<=).