LOOKAROUNDS
Lookahead
(?=...) peeks forward without consuming. (?!...) demands absence.
A lookahead is a condition, not a match. \d+(?=px) matches digits ONLY if "px" comes right after, but the px stays out of the match. In "10px" it matches "10" and leaves the px alone.
\d(?=px) positive lookahead: a digit only if "px" follows. In "5px" matches the 5; in "5em" matches nothing.\d(?!px) negative lookahead: a digit only if "px" does NOT follow. In "5em" matches the 5; in "5px" matches nothing.Because lookarounds consume nothing, they let separate conditions stack on the same spot: ^(?=.*\d)(?=.*[a-z]) asserts a line contains a digit somewhere and a lowercase letter somewhere - the classic password check shape. To assert what comes BEFORE a position instead, reach for lookbehind.
PRACTICE - 2 DRILLS 0/2 DONE
Match every number that is immediately followed by px. The px itself must not be part of the match.
/ /
width: 10px; top: 5em; gap: 3px
must match: "10" "3"
size 20pt
must match nothing
8px 9px
must match: "8" "9"
Match foo only when it is NOT immediately followed by bar.
/ /
foobar foobaz foo
must match: "foo" "foo"
foofoo
must match: "foo" "foo"
foobar
must match nothing