Andrew Johnson wrote:

> Essentially, they are the same as look-aheads ... zero-width assertions,
> except that the look-behind expression must be a fixed width pattern (no
> indeterminate quantifiers), and no captures are allowed in a negative
> look-behind 

So it is implemented as zero-width look-ahead + eating as many 
characters as the content matches?

(I've thought about implementing /foo/.preceded_by('bar') as 
/(?!bar).{3}foo/.)

> regards,
> andrew

More regards,
Florian Gross