On JavaScript regular expression non capture grouping

Non-captive grouping defines that a sub-expression can be modified as a whole but that the sub-expression matching results are not stored.

Non-captive grouping by placing subexpressions in "? : "After the symbol.

str = "img1.jpg,img2.jpg,img3.bmp";

reg = /(?:\w*)(?=\.gif)/;

arr_m = str.match(reg);//arr_m = ["img1","img2"]

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