How i replace the content between begin{document} and end{document} - livecode

Question

How i replace the content between begin{document} and end{document}. while using my code the whole content are replaced.In below code i am using wordOffset but it's not working
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put wordOffset("begin{document}",fld "MytextField") into tBegin
put wordOffset("end{document}",fld "MytextField") into tEnd
put the htmlText of field "MytextField" into myHtml
--put wordOffset("begin{document}",fld "myHtml") into tBegin
--put wordOffset("begin{document}",fld "myHtml") into tEnd
set the caseSensitive to true
put the field SRText into myArrayToBe
split myArrayToBe by CR
--enable the field "SRText"
--put "red,RED" & CR & "green,GREEN" & CR & "blue,BLUE" into myArrayToBe
--split myArrayToBe by CR
put the number of lines of (the keys of myArrayToBe) into myArraylength
repeat with i = 1 to myArraylength
--return i
put myArrayToBe[i] into y
split y by colon
put y[1] into searchStr
put y[2] into replaceStr
if searchStr is empty then
put the 0 into m
else
--put (word tBegin to tEnd of fld "MytextField")
--put replaceText(word tBegin to tEnd of fld "MytextField","searchStr","good") into word tBegin to tEnd of fld "MytextField"
put wordOffset("begin{document}",fld "MytextField") into tBegin
put wordOffset("end{document}",fld "MytextField") into tEnd
--put holder into myHtml
--put replaceText(word tBegin to tEnd of fld "MytextField",searchStr,replaceStr)of fld "MytextField"
replace searchStr with "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & searchStr & "</font></strike><font bgcolor=" & quote & "green" & quote & ">" & replaceStr & "</font>" in myHtml
end if
end repeat
--enable me
set the htmlText of fld "MytextField" to myHtml
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Answer 1

You need to use htmlText to both get and set the text, otherwise your offsets are off (e.g. if the first line of the field is bold, htmlText would give something like
<b>foo</b><br>begin{document}
whereas just 'fld "MytextField"' would give
foo
begin{document}
so offset("begin{document}",...) of the first gives 16, while for the second it gives 5. What I'd do is something like
put the htmlText of fld "MytextField" into theHTML
put offset("begin{document}", theHTML) into tBegin
put offset("end{document}", theHTML) into tEnd
add length of "begin{document}" to tBegin
put character tBegin to tEnd of theHTML into textToChange
-- now do your search/replace on textToChange, not the field
put textToChange into character tBegin to tEnd of theHTML
set the htmlText of fld "MytextField" to theHTML
I use offset() here, which gives a character offset, but wordOffset as you had it should work, too, if you keep in mind that it gives you word numbers, so you'd have to replace word tBegin to tEnd of theHTML.