# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(<blockquote.*?class='instagram-media'[^>]*>)(.*?)(<\\/blockquote>)/\1<\/blockquote>/im;t' <<< "<blockquote class='instagram-media' data-instgrm-captioned data-instgrm-permalink='https://www.instagram.com/p/BjaT3cgBHp4/' data-instgrm-version='8' style=' background:#FFF; border:0; border-radius:3px; box-shadow:0 0 1px 0 rgba(0,0,0,0.5),0 1px 10px 0 rgba(0,0,0,0.15); margin: 1px; max-width:658px; padding:0; width:99.375%; width:-webkit-calc(100% - 2px); width:calc(100% - 2px);'><div style='padding:8px;'> <div style=' background:#F8F8F8; line-height:0; margin-top:40px; padding:50.0% 0; text-align:center; width:100%;'> <div style=' background:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACwAAAAsCAMAAAApWqozAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAAMUExURczMzPf399fX1+bm5mzY9AMAAADiSURBVDjLvZXbEsMgCES5/P8/t9FuRVCRmU73JWlzosgSIIZURCjo/ad+EQJJB4Hv8BFt+IDpQoCx1wjOSBFhh2XssxEIYn3ulI/6MNReE07UIWJEv8UEOWDS88LY97kqyTliJKKtuYBbruAyVh5wOHiXmpi5we58Ek028czwyuQdLKPG1Bkb4NnM+VeAnfHqn1k4+GPT6uGQcvu2h2OVuIf/gWUFyy8OWEpdyZSa3aVCqpVoVvzZZ2VTnn2wU8qzVjDDetO90GSy9mVLqtgYSy231MxrY6I2gGqjrTY0L8fxCxfCBbhWrsYYAAAAAElFTkSuQmCC); display:block; height:44px; margin:0 auto -44px; position:relative; top:-22px; width:44px;'></div></div> <p style=' margin:8px 0 0 0; padding:0 4px;'> <a href='https://www.instagram.com/p/BjaT3cgBHp4/' style=' color:#000; font-family:Arial,sans-serif; font-size:14px; font-style:normal; font-weight:normal; line-height:17px; text-decoration:none; word-wrap:break-word;' target='_blank'>11 days after the #RoyalWedding & Iām still tryna find someone to hold my hand and do the thumb thing like this š©š©</a></p> <p style=' color:#c9c8cd; font-family:Arial,sans-serif; font-size:14px; line-height:17px; margin-bottom:0; margin-top:8px; overflow:hidden; padding:8px 0 7px; text-align:center; text-overflow:ellipsis; white-space:nowrap;'>A post shared by <a href='https://www.instagram.com/accessonline/' style=' color:#c9c8cd; font-family:Arial,sans-serif; font-size:14px; font-style:normal; font-weight:normal; line-height:17px;' target='_blank'> Access</a> (@accessonline) on <time style=' font-family:Arial,sans-serif; font-size:14px; line-height:17px;' datetime='2018-05-30T18:09:36+00:00'>May 30, 2018 at 11:09am PDT</time></p></div></blockquote>"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html