Building on Mr.Black and Roberts126 answers:

/(&lt;img(?!.*?alt=(['"]).*?\2)[^&gt;]*)(&gt;)/


This will match an img tag anywhere in the code which either has no alt tag or an alt tag which is not followed by ="" or ='' (i.e. invalid alt tags).

Breaking it down:

(          : open capturing group
&lt;img       : match the opening of an img tag
(?!        : open negative look-ahead
.*?        : lazy some or none to match any character
alt=(['"]) : match an 'alt' attribute followed by ' or " (and remember which for later)
.*?        : lazy some or none to match the value of the 'alt' attribute
\2)        : back-reference to the ' or " matched earlier
[^&gt;]*      : match anything following the alt tag up to the closing '&gt;' of the img tag
)          : close capturing group
(&gt;)        : match the closing '&gt;' of the img tag


If your code editor allows search and replace by Regex you can use this in combination with the replace string:

$1 alt=""$3


To find any alt-less img tags and append them with an empty alt tag.  This is useful when using spacers or other layout images for HTML emails and the like.

Using regular expressions to find img tags without an alt attribute
I would like to know how the signals for remote controlled cars, radios, etc.. That use radio waves are told apart from each other. I know that the radio waves are modulated to encode data and the frequency or amplitude are changed, so then the waves are propagated through the air and received at another location via a receiver that is tuned to a certain frequency that the waves were emitted, but I'm sure in most places in the world by now there are numerous amounts of waves traversing at any point, why doesn't the receiver of this device happen to catch another wave of the same frequency instead of the one that was intended? or is there anything that stops me from having a device that emits a wide range of frequencies or amplitudes that would manipulate nearby electronics?
