13.2.17 Lookbehind

Positive lookbehind (?<=) checks that its subpattern could match immediately to the left of the current position in the text string.

(pregexp-match-positions "(?<=grey)hound"
  "the hound in the picture is not a greyhound") 
 ⇒ ((38 . 43))

The regexp (?<=grey)hound matches hound, but only if it is preceded by grey.

Negative lookbehind (?<!) checks that its subpattern could not possibly match immediately to the left.

(pregexp-match-positions "(?<!grey)hound"
  "the greyhound in the picture is not a hound")
 ⇒ ((38 . 43))

The regexp (?<!grey)hound matches hound, but only if it is not preceded by grey.

Lookaheads and lookbehinds can be convenient when they are not confusing.

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