Powershell: Combining the -replace operator with a lookbehind assertion slowed down by a factor of 30+ between v6.0.2 and v6.1.0-preview.2

[mklement0]

Note: I have not tried to analyze whether the problem is in PowerShell or CoreFx; on the PowerShell side, new features were recently added to -replace - see #6029 - but I have no idea whether this symptom relates to that.

The problem appears to be specific to -replace; using -match does not exhibit the symptom.

Steps to reproduce

Run the following command line:

• first on v6.0.2 (the current stable release as of this writing).

• then on v6.1.0-preview.2 (the current preview release).

$arr =  (, 'fooo') * 1e6; (Measure-Command { $arr -replace '(?<=.{3}).+' }).TotalSeconds

Expected behavior

Similar timings.

Actual behavior

Here are sample timings, in seconds, from a Windows 10 VM:

# v6.0.2
1.8444752

# v6.1.0-preview.2
# !! more than 30 times slower
61.5428622

Environment data

PowerShell Core v6.0.2
PowerShell Core v6.1.0-preview.2

[SteveL-MSFT]

@mklement0 this is a great find. I did a private build reverting the change in #6029 and the same test on my macBook went from 35.6s to 1.4s so that PR is causing this perf degradation.

[rkeithhill]

Well, that's an unfortunate perf regression. You'd think that the PR could treat a replacement string the way it originally worked and only invoke the new code if replacement is of type scriptblock.

[BrucePay]

Note: the regression is not related to the actual regex, the operator was slow for any replacement string because it went through a very expensive path testing for MatchEvaluator before defaulting to string.

[daxian-dbw]

Closed via #7135