(?si)applies the following effective flags to the remainder of the pattern: gsi s modifier: single line. Allows a dot to match line terminators
i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters
\s*any whitespace character, repeated any number of times
<imgthe literal text <img (case insensitive) \ban ASCII word boundary
Atomic Group(?>(?!src=).)*? *?repeats the group contents below any number of times:
Negative Lookahead(?!src=) src=the literal text src= (case insensitive) .any character
\/the literal / (4710 / 578 / 2F16)
>the literal > (6210 / 768 / 3E16)
\s*any whitespace character, repeated any number of times
g modifier: global. Finds all matches instead of stopping after the first