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How to extract string from file, run filter, and replace in file with new value?

**TASK**
I am coding up ebooks to a specific standard, and have a script that converts a string into the correct titlecase for this publisher. When working with some public domain source files, one often gets this for a chapter title string:
&emsp;&emsp;&emsp;`HERE IS MY TITLE`
Using VSCodium (FOSS VS Code alternative), I can open each file, select the string between the `p` tags, then run the `titlecase` script with a hotkey that I've assigned it to. I end up with
&emsp;&emsp;&emsp;`Here Is My Title`
(VSCodium's native titlecase filter isn't up to this job.) I save the file, and go on to the next one.
If you only have a few of these to do, that's fine. But sometimes there can be dozens, and it gets very tedious.
**QUESTION**
Is there a way that I can script this? I have scratched my head over both `awk` and `sed`, thinking that these are my prime options. But (as a rank amateur) I cannot work out how to:
- iterate through all `chapter-*.xhtml` files in a directory,
- extract my string (ALWAYS line 12 in the file, only string on line, between `...` tags),
- **run my "external" `titlecase` filter on that string**,
- replace the new string for the original one in the source file,
- for all those files. :)
(The step in bold is the one that is my biggest stumbling block.)
~~I would be grateful for any help with this! If I've omitted any relevant information, please say and I'll remedy that a.s.a.p.~~
~~David / Fife, UK~~

**TASK**
I am coding up ebooks to a specific standard, and have a script that converts a string into the correct titlecase for this publisher. When working with some public domain source files, one often gets this for a chapter title string:
&emsp;&emsp;&emsp;`HERE IS MY TITLE`
Using VSCodium (FOSS VS Code alternative), I can open each file, select the string between the `p` tags, then run the `titlecase` script with a hotkey that I've assigned it to. I end up with
&emsp;&emsp;&emsp;`Here Is My Title`
(VSCodium's native titlecase filter isn't up to this job.) I save the file, and go on to the next one.
If you only have a few of these to do, that's fine. But sometimes there can be dozens, and it gets very tedious.
**QUESTION**
Is there a way that I can script this? I have scratched my head over both `awk` and `sed`, thinking that these are my prime options. But (as a rank amateur) I cannot work out how to:
- iterate through all `chapter-*.xhtml` files in a directory,
- extract my string (ALWAYS line 12 in the file, only string on line, between `...` tags),
- **run my "external" `titlecase` filter on that string**,
- replace the new string for the original one in the source file,
- for all those files. :)
(The step in bold is the one that is my biggest stumbling block.)

scripting sed text-processing