c++11 regex lookahead exclude word

I have a string list:

1. ReferencePrice
2. ReferenceCurrentPrice
3. CostPrice
4. AverageCostPrice ...

I want to filter out all strings that:

1. Containing 'Price',
2. But not containing 'CostPrice'

My regex is '(?!Cost)Price', but it can match the 3rd string 'CostPrice', why? and what is the correct regex?