switch -regex recompiles the clause pattern on every successful match #27975

# 1. Build a 200,000-line log.
$log = Join-Path ([IO.Path]::GetTempPath()) 'switch-regex-bench.log'
if (-not (Test-Path $log)) {
    $sb = [System.Text.StringBuilder]::new()
    for ($i = 1; $i -le 200000; $i++) {
        $null = $sb.AppendLine("2026-09-04T00:00:0$($i % 10) ERROR thing $i failed")
    }
    Set-Content -Path $log -Value $sb.ToString() -NoNewline
}

function Measure-Median ([scriptblock] $Action, [int] $Count = 5) {
    $times = foreach ($i in 1..$Count) { (Measure-Command $Action).TotalMilliseconds }
    ($times | Sort-Object)[[int]($Count / 2)]
}

# 2. Scenario A: one clause, every line matches.
$a = Measure-Median {
    switch -regex -file $log {
        '^(?<ts>\S+) ERROR (?<msg>.*)$' { $null = $matches }
    }
}

# 3. Scenario B: 23 non-matching clauses ahead of the matching one, so 24 distinct
#    patterns are evaluated per line -- more than [regex]::CacheSize, which is 15.
$clauses = (1..23 | ForEach-Object { "'NOMATCH{0:d2}(?<a>x)' {{ continue }}" -f $_ }) -join "`n    "
$b = Measure-Median ([scriptblock]::Create(@"
switch -regex -file `$log {
    $clauses
    '^(?<ts>\S+) ERROR (?<msg>.*)$' { `$null = `$matches }
}
"@))

# 4. Allocation cost of scenario A.
[GC]::Collect(); [GC]::WaitForPendingFinalizers(); [GC]::Collect()
$before = [GC]::GetTotalAllocatedBytes($true)
switch -regex -file $log { '^(?<ts>\S+) ERROR (?<msg>.*)$' { $null = $matches } }
$mb = ([GC]::GetTotalAllocatedBytes($true) - $before) / 1MB

'Scenario A : {0:N1} ms  (1 clause, median of 5)'   -f $a
'Scenario B : {0:N1} ms  (24 clauses, median of 5)' -f $b
'Ratio B/A  : {0:N1}x'                             -f ($b / $a)
'Allocated  : {0:N1} MB (scenario A)'               -f $mb

Name                           Value
----                           -----
PSVersion                      7.6.5
PSEdition                      Core
GitCommitId                    7.6.5
OS                             Arch Linux
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.4
SerializationVersion           1.1.0.1
WSManStackVersion              3.0