Consider the following regular expression:
https://regex101.com/r/svOSnY/1
I am trying to match out the memory amount and type form a Laptop HP Chromebook 14 G3 NVIDIA Tegra SOC 4GB DDRL 32GB FLASH 14inch 1366X768 Webcam Chrome OS, only, if it is not followed by an additional thing that looks like a memory amount. I thought that negative lookaheads are for exactly this reason:
(?!\d+\s?(gb|tb)) this is my negative lookahead
As it is applied now:
/(?:\d+)\s?(?:gb|tb)\s?(?:ddrl|ddr2)\s?(?!\d+\s?(gb|tb))/i
the 4gb ddrl part is still matched from my string, even though it is followed by the 32gb part which my negative lookahead should realise. If I change my negative lookahead, to a simple capture group, my regex correctly captures the whole 4gb ddrl 32gb part from the string.
What am I doing wrong?
Since you are declaring the space as optional, the regex egnine will try to match the string without considering the space; and indeed 4GB DDRL is not directly followed by 32GB FLASH (therefore it will be matched).
In order to fix it, put the optional space in your lookahead:
(?:\d+)\s?(?:gb|tb)\s?(?:ddrl|ddr2)(?!\s?\d+\s?(gb|tb))
See demo.