<imgthe literal text <img (case insensitive) \s+any whitespace character, repeated at least once
srcthe literal text src (case insensitive) .+any character (except for line terminators), repeated at least once
^start of line
Negative Lookahead(?!alt) altthe literal text alt (case insensitive) .+any character (except for line terminators), repeated at least once
g modifier: global. Finds all matches instead of stopping after the first
m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string
i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters