ICU4X should have a SimpleFormatter class like ICU4C that handles patterns like these. Can you make a follow-up issue and put in a // TODO(issuenumber)?

Originally posted by @sffc in unicode-org/icu4x#202 (comment)

I could imagine consumers of this library wanting to ignore fixed periods when getting the string for a day period in certain locales. Although I wouldn't necessarily advise this usage, someone might prefix the English language day period with the word "Good" as a greeting to users ("Good morning", "Good afternoon", etc.). Without filtering on fixed periods, this would lead to awkward construction

This should work:

cat test.txt | unic-inspector