This question already has an answer here:
I am trying to match base64 image strings using preg_match, I keep getting the error
Warning: preg_match(): Unknown modifier '/' in {filepath} on line 3
<?php
$html = "<p><img src=\"data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wCEAAkGBxISEBUSEhAVFRUVFQ8VFRUVEA8VEBUPFRUWFhUVFRUYHSggGBolHRUVITEhJSkrLi4uFx8zODMtNygtLisBCgoKDg0OFxAQGi0eHx8tLS0tLS0tKy0vKy0tLS0tLS0tLS0tLy0tLS0tLSstLS0tLS0tLS0tKy0rLS0rLS0rK//AABEIALcBEwMBIgACEQEDEQH/xAAbAAACAwEBAQAAAAAAAAAAAAACAwABBAUGB//EADcQAAICAQIDBQYEBgIDAAAAAAABAhEDBCESMUFRYXGBkQUTFCKhsULB0fAyUmJy4fEGojOCkv/EABoBAAMBAQEBAAAAAAAAAAAAAAABAgMEBQb/xAApEQACAgICAgEEAAcAAAAAAAAAAQIRAyESMQQTQRQiUWEyUoGh4fDx/9oADAMBAAIRAxEAPwD5nMUxkmKkdh5MQJC2HIFsRqgS6IEgGQtIlBIYmQoIECSFohaQxloKi0i6GIGg0ig0gEXFBxREg4oDRESCUQoxGRiAwVENQGQgHGIwsU4g8I+cQVEB2AomnToWoj8C3AjI9HRxR2KURuOPyhKBrejwPInTozS5mvSZDJlW4emdMhmM4XCzuQybC9RlpCFlMupz2ZxRx48LlKkIzTtmaQcpAUW2fRePgUIgNMhdFiOrieXkLkHJi2ZjQEgAmQDRFUWQsALRaKQQyWRlEZaACBwiCMxoYBLkVQxIFgAKDigUh0IgNIKMRsYlxgNjEBtgxiMjEtRGwgBLkVGI2MAoQHQgUYvJRmyRBUR+SJSgI0U9ClEfijuUoj8UQInk0bcS2GuOwOFDsipFTlR85lnc2c3JzJjdEZBWd0cfKCQx5BM5AtlMVnbg8eMCiFpF0B1FUQuiDA8gwJBSBZkWgCFlgUVRCyDERFkLARQSIkEhgUOxxAih8EAMlAMdJCpANFRRqxxE4VubcMAQN0XGI6MS4wGxiM5p5UgFEbGJSiacWKwRy5fJpAwgOjjNGLAaHg2LtI87J5Db7OTNblUMnHcnASzsXkUkgYodjiVDGa8OIXJIwy+Q2h+njsDq50hy2RztTltmHLlI48MHkmJBkyNlG1nv4oUiUQhAs3shCFjKIUWQAPGtglspEFELRVFgMjIQgCJRZC0hgXFBURIsLAKCNMEJxo040BMwMiFNDpi2gsqPQ3TR3OhigI0mM6OLHsP4OLPnq0BCA6OIdjxjlAiU6PMnltmeGI6Om04vHA6emiqM3kOTPldF4NMN1OKos14WjH7UzqqRPNtnBGUpTSOGsZfux6iFGA3kPQcwMWI0xjRNktzBqtd0RHJz6JhCeV1EPWajojA5AORDeMeKPb8fx1iQVlgl2VZ1pFosFFgUkWQhdDsZCBUQBniSEogiiEIWAEIQtIALSLSCoiAQUUWkWkHGIDRcYmnGtgIxNEYbCJkIkgFHc0SiBjjuDKukdHS4qjZstJIRllw44i8crYrs8OVzuTN8JjOMVGGxl1OWmZTV6Riocno62LcfHI0cnTamjbDUoyaaMMmJpm5ZpUL4G+Yh61CMvtHsElJmccUvhHQ4EuYnNrIx5HJzayT6ibNI4f5jrw+Hyf3s06jVuXgIKojN1SVI9TFjUFUUWiwUEkM6KLRaREgkhWUiJBJESCSCxlJBUWkEkFgVRAiBYHhSUXRKGWCFe1V279en78yUXQCKSGQiUkPhHYCXKgGiorcKQ3S47YEuVK2VwjcUBixbj8OEnkjKWekBDGaViNOn0jZq+D2J9iOWfmJaOVOAGKBtz4aMzQ+Vmq8jlEZrcv8ACuxF6LeRkyMbpM3C7F0jKUKx0ju5aUTkZd5Nh59daMvvTLGpds58OKUVbNWMemc74gv4hltMp4mzdKYiUxDyMpMpIqOMcmFYuIaHZ0QUUGmWikEhWbplpBFJlphZathJBJA8RfEBaiw0gkLsKxmigGSwLLGUsYdkBIBXrPArNLqi/ia5odwPrt40yp8S5tPvpcidiuL+Co54vr9GNhNPk0ZGr7K7UKlia7/UOTH64v5OrFDcjpUcvTZJLk77ndUbIZ0/4tn9B8rMZ4mmMSO/7L0D4HKjgYtTBNOT2vsZ7LF7TwR064cuNt9OON+a5nJ5ed44pRW2ed5zyRSUU9mWGh25GrDoTND2jKTpVR0NNxS6nFPNOKubo83K8kV92jfpdIqG5tOuFgYduo3UZVwczkflpS7PNk5cjiajEc/Pgo16rVJS5g59TFwu9z0ceWSo9PHzVHEzcxaZeSW4MWehZ6i6GMoFstByFQSRcUUi0xchMckQX70rjDYljkxyYUWIUi1IdHRDxvyaVIvjEKQSY6OmOJIdxBWKQaGbKCDTDQEUGkBaiEgkSKDSAqikgki0gkgsYNEGcJAsD5jHLKPKT9bNuLWqT4WvN+Bg25vfusG+wzTaKljjI68IQa4o+v8AslNPmvOjmafM4uuKr/pbVjMuXfmny/DRXIweF32dLwe/cVkhfQxaXNFN3t2XyNbyc7jyre+3tQ0zOUGmJenlzpMuOntcqH45cVtNoON1uOgeSS0ZtNlnjl8k2ufXp4dTr4v+RamFbxfb8n3pmBpPa9/AJx23ZjkwY8mpJMzyLHk/jin/AEOrD/lE3/Eq6fLuvqXl9up7e89VI4qxfvYqcY9TJeFhXUUjD6TBeo0dKetj/OvUKOW1zOPwLoXGLXJtG3q1ov6ePwzqyZcZI5scs+2/Gi3qnyr7oKY147Z0XkQPvTGtSu/0DjkT5MpIa8dLs0e8JxibLsqiljih3EWpCrCTGXSHJhxYlMZFgWh0WMiKixkQLQ2IyIER0UBSLihkUVFDYxFZSRIoNIuMTH7Q9q4sO0ncqvhirlX2XmIo20EkcDJ/yfHwS4YS4/wxlXC33tM5cvbuqkucYf2xS+rtgJtI9okWfOcmtyt377J5SnRBBZnWjfWUV5ip40qSfFz5WNx6xp/NFPv5P1NMdZHomr57Lb0FoTc18WYlKSTVOtr2+l9gvhZ0ZZcblvkW3bDYqWDE3fvKvomvzHQeyu1/Y5/A+g7HlcWmpX3fNVdm5oySUflgrp3+FpmZaWb/AAvz2FRSkn2aMmqVpwTXan9u804tVF3vXjRydwoSaaafmCk0TLDFo6OTOqbi3J9nTxFvUN7ySj57vyGvG5Q2a/uT4PWxOP5NsifdsqGzOKjX+2DHVb9nY/1NCnJrmn03rb9THqYw5r0TtfbYkI/K386rdPoTv4Zrxg1tG3S6hJ/Or8uoPxDt3SvlsmjM89reO7rq7ffyG4oca23++xLb7ZahH4QvPqJN9ldnIbhy7q+XPfqAo7NfvuJPHw/ddyfMV2VxSDlJN/Ld/Y0QafTyox7PrVdepfH2t7L1Gm0ROCa0b/evs+pfxC7PsYZt9H0fgKeSXNmnI5/SzpfFLlT9BmPOnyfl19DkSm3+q6mnHcluvMdiljpHUUxkchzcLklu/wDQWOc09pX3OKf1Kshd0dWEzRj8Dkx101zUPR/qFk9sSj+GPo3+YWWvwegxYGzRLTqMeKU4xS5t7JeZ5Re28zW0kv8A1Vox5dRPJvOTfY2/suhLNeSR28/t/esUL/qlaXkv1MWq9qZ5qnPgX9Hyv1u/qYFJ1tt39RUabrilfmwM+TZ0oe18qVe/fm4t+r3Odr87cuLaUpbt3bsdHSf1L0FZMUk6UfNVQMUZK+zMnPpz+ozilycb8ldG/HCuy/Ci6XmNRE8y/BmgoVyfoyGr3ZY+Jn7EcacXF01v6kxpt0lbZo+GVW5pvevm28e8bouCNy4raW+zpIzSOt5KjrZePQKvmu+6inoYxdyace9tP/Ix+0Yd/ojFqtRxO0q89qLfEyh7W96RpnqIqFY6T70uKu4we8vqRvtK27PqQ3Z0Qgol2iOLXNNWSNGqOeUlwLfp05dwUDbXRlxz6HRwaramlLss58sai2pc+1OwEw6FOCmdbVYMbjxRlGPltfpZjwKDdSk0u23TEtOioz7Btkxg1GrHZ5q+HiuP9K2/2VptQ4NOD59vaKkr3WwCRL2aRVHRefiXFKLtXyTppcwoS8Gn380ZdJkp05fK+e2z8TZm00Gri/Bq3y8BevWiZZuMqaE58e9xT36GeSY/SzVtOW+/NSG8SupOD59Oo1EHlp1RiizTjxykulLbpfgaJZIpXUfN1+RbzPZqFrtUkVxMZZW+kDpMa/lrx6mrhS/ewmWWXSHq0KcHJVNJ+DpFGV27ZolFAvGJx5ccNr4e5uw/iofzfR0NNEuMr0i3hb6/T8yvhE/9lx1EP516hcafKcf35hSFc1+ie4SWyRlzaSbd8QWfPTriT/tX3e4jJOXbL1f5pCbRrjjNbs0RxS6hQi+yvQx/EtbNu14Fy1ClzlJeFV9BWivXJm9MOMzLixTq4y4k+kkOx5t6lFx7/wAPqWmYSj+NjVIkvAPhK4SjO0BZA+Aggsye5hCO6W3VpW2c2eROXyxpdgep1Ll4dnYIRk3Z6OKDW5PYyMaAlzDjO1RTQi1+wZA2WuYaSfYIoWFxEcStt78gAkYt8hscTr98hcMjXIKOZ9QE7CjLaily3KyVzVotZlW6AVF8NFyihan38wgCiJGz2fKO8Xs+jTavu7DJZO8aJlHkqOpOD6OflGKfqZ1iy3081G/Mfo9VxJLql6mkukzjc5QdNGfFpefE7vp08Nx0FGOy27ugUlsYMkMi/Gq7duXePoSufbNaTbdu13Kmv1F5IpS/8lPsdPfwMkc87pTt9nTyYz4O3du+9OxXZfDi9sblTVLiTvq4/ejPknXVPw2Q5YsnSQvJhydil5R+gmVGvyiY5x2bf/a16MbGre8afck/VGKHOq8rr7h6i10i+9WvIVluG6Hww47q732+bc0xtcvmXe1Zy45K5wj9f2g56p38jY00KWKT/wAm3LOMueO//ltCp6bHfKUb8KNGJNreUW/7Rc8nNOVd9tL6oZlFtaRqxSiklxRfml9Bvgr9DEpQa3kn5K/p+hMS/kUq7nHh+5XIyeM1vJ/TL0v7FQyJ9GvFNEWSXVet/lYy33eo0ZtV/wBFScr2jfmQZb7PsQAv9HmuZRRDE9goNPYhBAVKikQgwCUXe/3Df8SaVLvdkIIQtLmFCiiDGNUu7oA4r92QgElSxdbLtpbkIJjRXEWns1+0yEAdBaXJwyT/AHXU7yiQhpA4vLXTL4RE1JbKEa8ef0IQpnLGVMyT0bVt1XOq28LW4yGqpbwddfmTLIQ9PR0xfsX3FRzRlb95Lfu/wUsi4qTTffFqyECzTglY3PwWlJfcHEsfJLv6/voQgzNR+3sd7u18rvrvfLxF5Fwq+Bd/L8yEKoxUnyoPHNtcWy7qELJFv5oxve+f2LISzWKVsv4Xs4fJNP1Bek4VtOS89iyA0R7JXRax5NuHLfiv8BvPOC+aKfen+RCDrVhF8pcWiL2lHsf0IQhHNnV9LjP/2Q==\" style=\"width: 275px;\"></p><p>This is a good post</p>";
preg_match("/\"data:image/(gif|jpg|jpeg|tiff|png);base64,.*\"/", $html, $matches, PREG_OFFSET_CAPTURE);
print_r($matches);
?>
</div>