How to replace html text in livecode - livecode

Question

Iam using this code for replace a word with the same word with strike. Its works fine.
replace Str[1] with "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & Str[1] & "</font></strike>" in myHtml
But when i replace it vice versa its not working. Iam using this code fore replacing
replace "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & Str[1] & "</font></strike>" with Str[1] in myHtml

Answer 1

Your code actually works for me. it strikes and colours the "def" on the first answerbut it is back to normal on the second. Maybe you have some issue with escaping the quotes?
put "abc def" into myHtml
put "def" into Str[1]
replace Str[1] with "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & Str[1] & "</font></strike>" in myHtml
answer myHtml -- "def" is yellow and stroke
replace "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & Str[1] & "</font></strike>" with Str[1] in myHtml
answer myHtml -- "def" is back to normal text

Answer 2

Working example with two Buttons and one text field:
Button 1:
on mouseUp
put the htmlText of field "MytextField" into myHtml
put item 1 of myhtml into str[1]
replace Str[1] with "<strike><font bgcolor=" & quote & "yellow" & quote & ">" & Str[1] & "</font></strike>" in myHtml
set the htmlText of fld "MytextField" to myHtml
end mouseUp
Button 2:
on mouseUp
put the htmlText of field "MytextField" into myHtml
put "abc" into str[1]
replace "<strike><font bgcolor=" & quote & "#FFFF00" & quote & ">" & Str[1] & "</font></strike>" with Str[1] in myHtml
set the htmlText of fld "MytextField" to myHtml
end mouseUp
and how I set my textfield
set the htmltext of field "mytextfield" to "abc,def"