Lookahead and lookbehind, backreferences, lazy versus greedy matching, named groups, catastrophic backtracking and flavor differences.
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- What does a positive lookahead assertion do?
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Asserts that what follows matches a pattern without consuming characters.- What is the syntax for a positive lookahead?
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(?=...)- What does a negative lookahead assertion do?
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Asserts that what follows does not match a pattern without consuming characters.- What is the syntax for a negative lookahead?
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(?!...)- What does a positive lookbehind assertion do?
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Asserts that what precedes matches a pattern without consuming characters.- What is the syntax for a positive lookbehind?
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(?<=...)- What does a negative lookbehind assertion do?
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Asserts that what precedes does not match a pattern without consuming characters.- What is the syntax for a negative lookbehind?
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(?<!...)- Key difference: lookahead vs lookbehind?
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Lookahead checks forward in the string, lookbehind checks backward.- What is a backreference in regex?
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A reference to a previously captured group in the same pattern.- How do you write a numbered backreference to group 1?
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\1- In pattern (a+)\1, what does \1 match?
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The same text captured by the first group, i.e., the same sequence of a's.- Name a common use case for backreferences.
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Matching repeated or doubled words, like finding the same word twice in a row.- If a group is captured multiple times, what does a backreference refer to?
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The most recent capture of that group.- What is a lazy quantifier?
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A quantifier that matches as few characters as possible while still allowing the overall pattern to succeed.
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