Positive Lookahead
regex_1(?=regex_2)
The positive lookahead (?=) asserts regex_1 to be immediately followed by regex_2. The lookahead is excluded from the match. It does not return matches of regex_2. The lookahead only asserts whether a match is possible or not.
For Example
1 | c(?=o) |
is matched with chocolate
Task
gooooo!
1 | o(?=oo) |
Negative Lookahead
regex_1(?!regex_2)
The negative lookahead (?!) asserts regex_1 not to be immediately followed by regex_2. Lookahead is excluded from the match (do not consume matches of regex_2), but only assert whether a match is possible or not.
For Example
1 | c(?!o) |
is matched with chocolate
Task
If S =goooo, then regex should match goooo. Because the first g is not follwed by g and the last o is not followed by o.
gooooo
1 | (\S)(?!\1) |
Positive Lookbehind
(?<=regex_2)regex_1
The positive lookbehind (?<=) asserts regex_1 to be immediately preceded by regex_2. Lookbehind is excluded from the match (do not consume matches of regex_2), but only assert whether a match is possible or not.
For Example
1 | (?<=[a-z])[aeiou] |
is matched with helo
Task
123Go!
1 | (?<=[13579])\d |
Negative Lookbehind
(?<!regex_2)regex_1
The negative lookbehind (?<!) asserts regex_1 not to be immediately preceded by regex_2. Lookbehind is excluded from the match (do not consume matches of regex_2), but only assert whether a match is possible or not.
For Example
1 | (?<![a-z])[aeiou] |
is matched with helo
Task
1o1s
1 | (?<![aeiuoAEIOU]). |