Regex Practice Lesson 10 of 14

Backreferences

Groups and capture (Phase 5) pull a piece of a match back out after the match finishes. A backreference, \1, reuses a captured group inside the same pattern - it says "whatever group 1 matched, match that exact text again, right here."

/\b(\w+)\s+\1\b/ reads as: capture a word, then require the same word again after some whitespace. "the the" matches - group 1 captures "the", and \1 demands another "the". "the cat" doesn't match - \1 needs "the" again, but finds "cat". You can't do this with a plain quantifier; \w+\s+\w+ would match any two words, not specifically two of the same word.

Your task: write hasRepeatedWord(str), true if str contains the same word twice in a row (separated by whitespace), and isDoubledString(str), true if the entire string is some sequence of word characters repeated exactly twice back-to-back (like "abcabc"), with nothing else around it.

You'll practice:

Reusing a captured group with \1 to require matching text
Anchoring a backreference to check the whole string, not just part of it

// Write hasRepeatedWord(str): true if the same word appears twice in a row.
function hasRepeatedWord(str) {

}

// Write isDoubledString(str): true if the whole string is some sequence
// of word characters repeated exactly twice back-to-back, e.g. "abcabc".
function isDoubledString(str) {

}