This is a "zero-width, look-behind assertion". The assertion is true if  (?<!/) does not immediately follow a '/' (forward slash) character at the point at which the assertion occurs in the regex.

See  (?<!pattern) in the Extended Patterns section (in the Look-Around Assertions subsection) of perlre.

Update: Added "zero-width" to description.

In reply to Re^3: Getting around "/" as a word boundary by AnomalousMonk
in thread Getting around "/" as a word boundary by sherab

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