I think the empty space is literally catching the space between words. The actual regex only has single slashes, it's double in the code because it needs escaped.

From regex101.com:

"(\b\S )( \1\b) "

gm 1st Capturing Group (\b\S ) \b assert position at a word boundary: (^{\w|\w$|\W\w|\w\W)} \S matches any non-whitespace character (equivalent to [^\r\n\t\f\v ]) matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)

2nd Capturing Group ( \1\b) matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy) A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data matches the character with index 3210 (2016 or 408) literally (case sensitive) \1 matches the same text as most recently matched by the 1st capturing group \b assert position at a word boundary: (^{\w|\w$|\W\w|\w\W)} Global pattern flags g modifier: global. All matches (don't return after first match) m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)