(?:...): grouping without creating a capture

The two brackets do the same grouping work. /(?:ab)+/ and /(ab)+/ match exactly the same text; the difference is that the second one also hands back the last ab it saw. If nothing is going to read that value, capturing it is pure overhead and, worse, it occupies a number.

That number is why this matters more than it first appears. Group numbers are positional, so wrapping something in parentheses halfway through a pattern shifts every later group up by one. Any $1 in a replacement string, any \1 backreference and any m[2] in the surrounding code now points somewhere else, and none of them raise an error about it.

The worked example below is the shape that bites. A date reformatter using $3/$2/$1 works correctly until someone wraps the month in an extra pair of parentheses for an unrelated reason. The pattern still matches, the replacement still runs, and the output is wrong in a way that looks plausible enough to ship.

The habit worth forming is simple: parentheses capture only when you intend to read the value. Everything else, particularly grouping around an alternation or before a quantifier, gets the non capturing form, and the pattern then survives being edited by someone who was not there when it was written.