<iframethe literal text <iframe (case sensitive) .*?any character (except for line terminators), repeated any number of times
src="the literal text src=" (case sensitive) Negative Lookahead(?![^"]*(?:youtube|vimeo|soundcloud)) Negated Character Class[^"]* *matches any number of characters outside the set below:
"the literal " (3410 / 428 / 2216)
Non-Capturing Group(?:youtube|vimeo|soundcloud) youtubethe literal text youtube (case sensitive) vimeothe literal text vimeo (case sensitive) 3rd Alternativesoundcloud soundcloudthe literal text soundcloud (case sensitive) .*?any character (except for line terminators), repeated any number of times
<the literal text < (case sensitive) \/the literal / (4710 / 578 / 2F16)
iframe>the literal text iframe> (case sensitive) g modifier: global. Finds all matches instead of stopping after the first