- True if a grouped expression is not ahead of the current position.
- Reject overall match if a group expression matches.
- Group is not included in the match or captured.
- Negative Lookahead Metacharacter is:
?! Group is a negative lookahead assertion.? modifies the group.! makes the group into a negative lookahead assertion.
- An example is
/(?!seashore)sea/ matches “sea” in “seaside” but not “seashore”.
- A second example is
/\b(?!re)\w+\b/ matches words not starting in “re”.
- It matches “cycle”, but not “recycle”.
- A third example is
/\b\w+(?!er)\b/ matches words not ending in “er”.
- It matches “run” and “running”, but not “runner”.
- Not merely the opposite of positive lookahead assertions.
- This rejects whole expressions, not just simple character matches.
- Can find “sales tax” instead of “sales” for example .
- A fourth example is “The green frog chased the green bug in the green grass.” We can use a word boundary such as
/\bgreen\b/, which finds all the words of “green”.
- To miss out the “green” before “frog” and capture the other words, we use
/\bgreen\b(?! frog)/, which captures the “green” before “bug” and the “green” before “grass”.
- To find only the last instance of “green”, which is “green” before “grass”, we can use
/\bgreen\b(?!.*green)/
- Do not match lines that begin with a code comment indicate
#.
- We can do this with negative lookahead assertion, which is
/^(?!\s*#).+$/