When searching multiple patterns, why doesn't -o always return the longest match?

[cbielow]

when combining -o (print hit) with -f (multiple patterns read from a file), the output changes, depending on the order of patterns in -f (if a pattern is a prefix of another pattern).

Example:

given:
(minidb.txt)

___PATTERN_EXT_____
___PATTERN_____

and a set of patterns:
(minip.txt)

PATTERN
PATTERN_EXT

then $ rg -o -f minip.txt minidb.txt
will report

1:PATTERN
2:PATTERN

but changing the order of patterns (i.e. PATTERN_EXT before PATTERN in -p) will find the longest match:

1:PATTERN_EXT
2:PATTERN

i.e. ripgrep may not report the longest match if a pattern is a prefix of another pattern and the prefix-pattern is listed before the longer pattern in -p.
Now, I could not find any docs on this, but grep does the "right" thing here and reports the longest match, independent of the order in the pattern file.

Version: ripgrep-14.1.1-x86_64-unknown-linux-musl

[BurntSushi]

This isn't a bug and it is the intended semantic. ripgrep provides no way to do what you want. A work-around, assuming your pattern file is just a list of literals (i.e., not regexes) is to sort your patterns in descending order of length. This guarantees that no preceding pattern is a prefix of one that follows it.

ripgrep only supports "leftmost-first" match semantics. You can find a description of that semantic in the library documentation for ripgrep's default regex engine. PCRE2 will behave the same way.

The upside of leftmost-first is that it allows you to express a preference order. The downside is that it isn't commutative, as you've discovered.

It is conceivable that, in the far future, ripgrep will support a flag to use leftmost-longest semantics while still supporting non-greedy operators (GNU grep's default regex engine only supports leftmost-longest, but not non-greedy operators). This is a large task and there are no immediate plans to work on it.

[cbielow]

thanks for the pointers to the relevant documentation.
I did not think this was a bug; I just wanted to mention it, since this may not be obvious to the casual user.

[BurntSushi]

Yeah sorry, for some reason I thought this was opened as a bug instead of a Q&A. My bad!