RegEx to exclude match string, but not if part of another string

Question

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.

Greg Sansom · Accepted Answer · 2012-12-14 05:08:38Z

2

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 (<=).

answered Dec 14, 2012 at 5:08

Greg Sansom

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2 Comments

Adding the \s* accounts for whitespace. So, (?<!grad\(\s*)vec is doing what I need. Thanks.

Well, it will break with vec*somefunc(vec), or funky stuff like vec*outerfunc(innerfunc(vec) * vec * factor, vec, somethingelse)