如果要在文件中插入自定义字符串的位置无关紧要，则可以执行以下操作：

$hash = [collections.arraylist]@()
[collections.arraylist]$out = switch -regex -file file.txt {
    'Microsoft .NET Framework (\d[\d\.]*)' {
        if ($hash -notcontains "[$($Matches[1])]") {
            [void]$hash.Add(("[{0}]" -f $Matches[1])) }
    }
    default { $_ }
}
$out,("Microsoft .NET Framework {0}" -f "$hash")

如果字符串的插入点很重要，则需要某种方法来确定该行号。有很多方法可以做到这一点。下面的代码一次读取文件，循环遍历文件的每一行，并跟踪包含第一个字符串匹配项的行号。

$hash = [collections.arraylist]@()
$firstIndex = 0
$data = Get-Content file.txt
[collections.arraylist]$out = for ($i = 0; $i -lt $data.count; $i++) {
    if ($data[$i] -match 'Microsoft .NET Framework (\d[\d\.]*)') {
        if (!$firstIndex) {
            $firstIndex = $i
        }
        if ($hash -notcontains "[$($Matches[1])]") {
            [void]$hash.Add(("[{0}]" -f $Matches[1]))
        }
    }
    else {
        $data[$i]
    }
}

$out.Insert($firstIndex,("Microsoft .NET Framework {0}" -f "$hash"))    
$out

不使用ArrayList的替代方法：

# read the file as string array
$lines = Get-Content -Path 'file.txt'
# create a regex string to capture the prefix and version
$regex    = '^(?<prefix>Microsoft \.NET Framework\s*)(?<version>[\d\.]+)'
$versions = @()
$count    = 0
$index    = -1

$result = $lines | ForEach-Object {
    if ($_ -match $regex) {
        if ($index -lt 0) {
            # remember the index of this line
            $index = $count
            # output just the prefix for now,we will add the versions to it later
            $matches['prefix']
            $count++
        }
        # store the version in an array
        $versions += $matches['version']
    }
    else { $_ ; $count++}  # no match,simply output the line and increase $count

}

$result[$index] += ($versions | Sort-Object -Unique | ForEach-Object { '[{0}]' -f $_ }) -join ' '

输出：

Other lines...
...
Microsoft .NET Core SDK 2.1.801 (x64)
Microsoft .NET CoreRuntime For CoreCon
Microsoft .NET CoreRuntime SDK
Microsoft .NET Framework [3.5] [4] [4.5] [4.5.1] [4.5.2] [4.6] [4.6.1] [4.7.1] [4.7.2]
Microsoft .NET Framework Cumulative Intellisense Pack for Visual Studio (ENU)
Microsoft .NET Native SDK
Windows Desktop SDK
...
Etc

如果要删除以Microsoft .NET开头的所有其他行，请将上面的代码更改为：

# read the file as string array
$lines = Get-Content -Path 'file.txt'
# create a regex string to capture the prefix and version
$regex    = '^(?<prefix>Microsoft \.NET Framework\s*)(?<version>[\d\.]+)'
$versions = @()
$count    = 0
$index    = -1

$result = $lines | ForEach-Object {
    if ($_ -match $regex) {
        if ($index -lt 0) {
            # remember the index of this line
            $index = $count
            # output just the prefix for now,we will add the versions to it later
            $matches['prefix']
            $count++
        }
        # store the version in an array
        $versions += $matches['version']
    }
    elseif ($_ -notlike 'Microsoft .NET*') { 
        # no match,output the line and increase $count if it does not start with "Microsoft .NET"
        $_
        $count++ 
    }   
}

$result[$index] += ($versions | Sort-Object -Unique | ForEach-Object { '[{0}]' -f $_ }) -join ' '

# output on screen
$result

Other lines...
...
Microsoft .NET Framework [3.5] [4] [4.5] [4.5.1] [4.5.2] [4.6] [4.6.1] [4.7.1] [4.7.2]
Windows Desktop SDK
...
Etc