^{Related: #7713 and #7867}

Sometimes all you want Select-String to do is to output only the matching parts of the input lines as strings, similar to what grep -o does on Unix-like platforms; e.g.:

# Extract only the parts that match the regex, each on its own line.
PSonUnix> "line1`nline2`nline3" | grep -o '[0-9]'
1
2
3

The equivalent Select-String solution is currently cumbersome:

PS> "line1", "line2", "line3" | Select-String '[0-9]' | ForEach-Object { $_.Matches[0].Value }
1
2
3

If we introduced a switch named, say, -MatchingPartOnly (name TBD, could have an alias of -o)
-OnlyMatching (see decision below), the command could be simplified to:

PS> "line1", "line2", "line3" | Select-String '[0-9]' -OnlyMatching

As an alias, -om could be considered (just -o could break existing code that used it for -OutVariable).

This would also speed up processing, because constructing [Microsoft.PowerShell.Commands.MatchInfo] instances can be bypassed.

Environment data

Written as of:

PowerShell Core 6.1.0-preview.4