You use a forward slash as the delimiter so you have to escape the slash here
Or use another delimiter like
Note: using the .* will also match
style=\"width: 275px;\ so you could make it non greedy
If you want to get the string inside the double quotes you could use
"(data:image\/(?:gif|jpg|jpeg|tiff|png);base64,.*?)" to capture the string in group 1.
Or to get a match without a capturing group you could use:
\K to reset the starting point of the reported match, then matches your string and use a positive lookahead to assert that what follows is a