Validate Text Alternative

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Version 0: For review

Algorithm to check validity of textual alternative. It must be non-empty, not a filename, an URL or a placeholder.

Check following for the result of the Text Alternative Computation algorithm:

Step 1. Alternative is non-empty

If the regular-expression "/([a-zA-Z0-9])+/ig" has less than three matches, return invalid.

Step 2. Alternative is not a filename

If the regular-expression "/.+\.(bmp|jpg|jpeg|jfif|gif|png|tif|tiff)/" matches, return invalid. (List of formats may be extended).

Step 3. Alternative is not a URI

Matching URIs by a regular expression is quite complex, but could be done. On match, return invalid.

Step 4. Alternative is not a placeholder

To identify placeholders, a blacklist (possibly of regular expressions) should be maintained to compare the textual alternative. On match, return invalid.

Example: {nbsp, spacer, bullet, image, photo, picture, placeholder, alt, …}

If none of the above issues is found, return valid.

Note: For 2 and 3 it may be simpler to check, if the textual alternative is contained in the value of the href or respectively in the value of the src attribute, because in most cases these invalid texts are inserted automatically by content management or portal systems.