powershell multiple block expressions - powershell

Question

I am replacing multiple strings in a file. The following works, but is it the best way to do it? I'm not sure if doing multiple block expressions is a good way.
(Get-Content $tmpFile1) |
ForEach-Object {$_ -replace 'replaceMe1.*', 'replacedString1'} |
% {$_ -replace 'replaceMe2.*', 'replacedString2'} |
% {$_ -replace 'replaceMe3.*', 'replacedString3'} |
Out-File $tmpFile2

Answer 1

You don't really need to foreach through each replace operations. Those operators can be chained in a single command:
#(Get-Content $tmpFile1) -replace 'replaceMe1.*', 'replacedString1' -replace 'replaceMe2.*', 'replacedString2' -replace 'replaceMe3.*', 'replacedString3' |
Out-File $tmpFile2

Answer 2

I'm going to assume that your patterns and replacements don't really just have a digit on the end that is different, so you might want to execute different code based on which regex actually matched.
If so you can consider using a single regular expression but using a function instead of a replacement string. The only catch is you have to use the regex Replace method instead of the operator.
PS C:\temp> set-content -value #"
replaceMe1 something
replaceMe2 something else
replaceMe3 and another
"# -path t.txt
PS C:\temp> Get-Content t.txt |
ForEach-Object { ([regex]'replaceMe([1-3])(.*)').Replace($_,
{ Param($m)
$head = switch($m.Groups[1]) { 1 {"First"}; 2 {"Second"}; 3 {"Third"} }
$tail = $m.Groups[2]
"Head: $head, Tail: $tail"
})}
Head: First, Tail: something
Head: Second, Tail: something else
Head: Third, Tail: and another
This may be overly complex for what you need today, but it is worth remembering you have the option to use a function.

Answer 3

The -replace operator uses regular expressions, so you can merge your three script blocks into one like this:
Get-Content $tmpFile1 `
| ForEach-Object { $_ -replace 'replaceMe([1-3]).*', 'replacedString$1' } `
| Out-File $tmpFile2
That will search for the literal text 'replaceMe' followed by a '1', '2', or '3' and replace it with 'replacedString' followed by whichever digit was found (the '$1').
Also, note that -replace works like -match, not -like; that is, it works with regular expressions, not wildcards. When you use 'replaceMe1.*' it doesn't mean "the text 'replaceMe1.' followed by zero or more characters" but rather "the text 'replaceMe1' followed by zero or more occurrences ('*') of any character ('.')". The following demonstrates text that will be replaced even though it wouldn't match with wildcards:
PS> 'replaceMe1_some_extra_text_with_no_period' -replace 'replaceMe1.*', 'replacedString1'
replacedString1
The wildcard pattern 'replaceMe1.*' would be written in regular expressions as 'replaceMe1\..*', which you'll see produces the expected result (no replacement performed):
PS> 'replaceMe1_some_extra_text_with_no_period' -replace 'replaceMe1\..*', 'replacedString1'
replaceMe1_some_extra_text_with_no_period
You can read more about regular expressions in the .NET Framework here.