According to http://www.rebol.com/docs/core23/rebolcore-15.html
You can use change/part to parse and replace but that doesn't work well with this where I just try to replace the block
<mytag > ... </mytag>
by "MyString"
content: {<mytag id="a" 111111111111111>
</mytag>
aaaaaaaaaaaaaaa
aaaaaaaaaaaaaaa
<mytag id="b" 22222222222222222>
</mytag>
<mytag id="c" 3333333333333>
</mytag>
aaaaaaaaaaaaaaa
aaaaaaaaaaaaaaa
<mytag id="d" 444444444444444>
</mytag>
}
mytag: [ to {<mytag} start: (
)
thru {<mytag}
to {id="} thru {id="} copy ID to {"} thru {"}
to {</mytag>}
thru {</mytag>}
ending:
(change/part start "mystring" ending)
mark:
( write clipboard:// mark
input)
]
rule: [any mytag to end]
parse content rule