Confused about 'positive lookbehind assertion'

[Robert Dailey]

Hi,

I've been reading the python documentation on 'positive lookbehind
assertion' and I don't understand at all how it works. The python docs give
the following example:

"**(?<=abc)def will find a match in "abcdef", since the lookbehind will back
up 3 characters and check if the contained pattern matches."

Can anyone emphasize more on what this RE operation does? Thanks.
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