I'm trying to search a .txt dictionary for all trisyllabic roots, and then have the matching roots passed to a new .txt file. The dictionary in question is a raw text version of Heath's Nunggubuyu dictionary. When I search the file in Atom (my preferred text editor), the following string does a pretty good job of singling out the desired roots and eliminating any material from the definitions below the headwords (which begin with whitespace), as well as any English words, and any trisyllabic strings interrupted by a hyphen or equals sign (which mean they are not monomorphemic roots). Forgive me if it looks clunky; I'm an absolute beginner. (In this orthography, vowel length is indicated with a ':', and there are only three vowels 'a,i,u'. None of the headwords have uppercase letters.)
^\S[^aeiousf]*[aiu:]+[^csfaioeu:\-\=\W]+[aiu:]+[^VNcsfaeiou:\-\=]+[aiu:]+[^VcsfNaeiou:]*\b
However, I need the matched strings to be output to a new file. When I try using this same string in grep (on a Mac), nothing is matched. I use the syntax
grep -o "^\S[^aeiousf]*[aiu:]+[^csfaioeu:\-\=\W]+[aiu:]+[^VNcsfaeiou:\-\=]+[aiu:]+[^VcsfNaeiou:]*\b" Dict-nofrontmatter.txt > output.txt
I've been searching for hours trying to figure out how to translate from Atom's regex dialect to grep (Mac), to no avail. Whenever I do manage to get matches, the results looks wildly different to what I expect, and what I get from Atom. I've also looked at some apparent grep tools for Atom, but the documentation is virtually non-existent so I can't work out what they even do. What am I getting wrong here? Should I try an alternative to grep?
Possibly the only thing missing from your grep command is the -E option:
regex='^\S[^aeiousf]*[aiu:]+[^csfaioeu:\-\=\W]+[aiu:]+[^VNcsfaeiou:\-\=]+[aiu:]+[^VcsfNaeiou:]*\b'
grep -Eo "$regex" Dict-nofrontmatter.txt > output.txt
-E activates support for extended (modern) regular expressions, which work as one expects nowadays (duplication symbols + and ? work as expected, ( and ) form capture groups, | is alternation).
Without -E (or with -G) basic regular expressions are assumed - a limited legacy form that differs in syntax. Given that -E is part of POSIX, there's no reason not to use it.
On macOS, grep does understand character-class shortcuts such as \S and \W, and also word-boundary assertions such as \b - this is in contrast with the other BSD utilities that macOS comes with, notably sed and awk.
It doesn't look like you need it, but PRCEs (Perl-compatible Regular Expressions) would provide additional features, such as look-around assertions.
macOS grep doesn't support them, but GNU grep does, via the -P option. You can install GNU grep on macOS via Homebrew.
Alternatively, you can simply use perl directly; the equivalent of the above command would be:
regex='^\S[^aeiousf]*[aiu:]+[^csfaioeu:\-\=\W]+[aiu:]+[^VNcsfaeiou:\-\=]+[aiu:]+[^VcsfNaeiou:]*\b'
perl -lne "print for m/$regex/g" Dict-nofrontmatter.txt > output.txt