Why replace operator matches empty string in powershell

Question

In Powershell (5.1 or 7), I run:

PS R:\> "abcdef" -replace '.*','x'
xx
PS R:\> "abcdef" -replace '.+','x'
x
PS R:\> "abcdef" -replace '^.*','x'
x
PS R:\> "abcdef" -replace '^.+','x'
x
PS R:\>
PS R:\> "abcdef" -replace '^','x'
xabcdef
PS R:\>

As you can see, in the first run I got xx but was expecting a single x. Tried with sed in bash (executables from gitdir/usr/bin; msys I think), and got what I expected.

2021-05-01 01:34:27 /r :
$ echo "abcdef" | sed -E s/.*/x/g
x

2021-05-01 01:35:03 /r :
$ echo "abcdef" | sed -E s/.+/x/g
x

2021-05-01 01:35:08 /r :
$ echo "abcdef" | sed -E s/^.*/x/g
x

2021-05-01 01:35:17 /r :
$ echo "abcdef" | sed -E s/^.+/x/g
x

2021-05-01 01:35:20 /r :
$ echo "abcdef" | sed -E s/^/x/g
xabcdef

2021-05-01 01:35:25 /r :
$

I have tried the documentation and cant figure out how to understand what is happening.

Answer 1

Let's find out!

The easiest way to find out what exactly was matched by a regex pattern in any version of PowerShell is by using Regex.Matches():

PS ~> [regex]::Matches('abcdef', '.*')
    
Groups   : {0}
Success  : True
Name     : 0
Captures : {0}
Index    : 0
Length   : 6
Value    : abcdef

Groups   : {0}
Success  : True
Name     : 0
Captures : {0}
Index    : 6
Length   : 0
Value    :

Aha! It's matching the substring abcdef, and then the empty string between f and the end of the string.

---

In PowerShell 7 we can also use a scriptblock with the replace operator to confirm:

PS ~> "abcdef" -replace '.*',{"['$($_.Value)' (length $($_.Length)) starting at $($_.Index)]"}
['abcdef' (length 6) starting at 0]['' (length 0) starting at 6]

I'm afraid I don't now why the regex engine implementors decided that this behavior was preferable to the behavior of sed, but at least we know what happens now.

Answer 2

Select-string showing 2 matches:

# select-string highlights matches in ps 7, but you can't see the 2nd match anyway
'abcdef' | select-string .* -AllMatches | % matches   # 2 matches

Looks like a .Net thing, even in Powershell 7. regex101.com/r/VzxbOT/1 gives 2 matches as well, so maybe it's sed that's wrong ("posix leftmost longest match?" Should .net follow that standard?), since the /g means global or all matches?

[regex]::Replace('abcdef','.*','x')

xx

Replace only one time (Replacing only the first occurrence of a word in a string):

$pattern = [regex]'.*'
$pattern.replace('abcdef','x',1)

x

Search and replace in awk in osx works the same as sed. Only works in bash for some reason. Oh you'd have to backslash the required doublequotes in powershell.

echo 'abcdef' | awk '{ gsub(/.*/,\"x\"); print }'

x