RegexdrillBackreference across lines: duplicate lines
A regex exercise from Regexdrill. Solve it with a pattern box with live highlighting in your browser, checked against hidden test strings, and it is checked the moment you submit.
The task
Each test is a block of several lines. Whenever a line is immediately repeated, keep one copy and remove every repeat that follows it. Lines are never empty. The anchors need to work per line, so use the m flag, and every run of repeats must be handled.
What this exercise teaches
^(.+)(?:\n\1)+$ with the gm flags and the replacement $1 matches a line plus every identical line right after it, and writes the line back once. Without the anchors, a short line like b could match the tail of ab.
A hint if you get stuck
Capture a whole line between ^ and $, then require a line break followed by \1, repeated one or more times. With the g and m flags this runs on every line of the block.
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