Replace and inserting strings within .txt files with the use of regex

[Νίκος]

On 8 Αύγ, 05:42, John S <jstrick... at gmail.com> wrote:
> If the 500 web pages are PHP only in the sense that there is only one
> pair of <? ?> tags in each file, surrounding the entire content, then
> what you ask for is doable.

First of all, thank you very much John for your BIG effort to help
me(i'm still readign your posts)!

I have to tell you here that those php files contain several instances
of php opening and closing tags(like 3 each php file). The rest is
pure html data. That happened because those files were in the
beginning html only files that later needed conversion to php due to
some dynamic code that had to be used to address some issues.

Please tell me that the code you provided can be adjusted to several
instances as well!