Prof. Powershell

Regular Expressions, Part 3

A last look at expressions, this time by trying to match on opposites.

• By Jeffery Hicks
• 09/04/2012

Sometimes we want to match on opposites. For example, use a regular expression to find everything that is NOT a word character. We know what will happen with this:

PS C:\> 'Power$h3|| is fun' -match "\w+"
True
PS C:\> $matches

Name      Value
----      -----
0         Power

The opposite is the same pattern but a capital W. Remember, I told you these patterns can be case sensitive:

PS C:\> 'Power$h3|| is fun' -match "\W+"
True
PS C:\> $matches

Name      Value
----      -----
0         ||

Those spaces are also "matchable". The class \s will match any whitespace:

PS C:\> 'Power$h3|| is fun' -match "\s"
True

The actual match value isn't much help because it is a space. But you might use this pattern as a validator:

$p=Read-Host "Enter a password"
If ($p -match "\s") {
   Write-warning "Your password has a space which I don't like"
}

What do you think we use to match on all non-white spaces?

PS C:\> 'Power$h3|| is fun' -match "\S+"
True
PS C:\> $matches

Name      Value
----      -----
0         Power$h3||

The regular expression matches up until it doesn't,which in this case is the first space. But notice it grabbed numbers and the special characters. This is a handy class when you want to match everything BUT spaces or tabs, but it is very broad.

The last "opposite" is with digits. Normally use use \d to match a number. Use \D to match non-numbers.

PS C:\> 'Power$h3|| is fun' -match "\D+"
True
PS C:\> $matches

Name      Value
----      -----
0         Power$h

The regular expression matched until it hit a number.

So let's see everything we've covered thus far is some more practical examples. Open up a PowerShell prompt and try these commands. Before you run the command, predict what type of result you will get. You might be surprised where you can use regular expressions.

get-service | where {$_.name -match "\d{2}"}
get-service [a-b]*
get-process [cw]*
dir c:\windows | where {$_.PSIsContainer -AND $_.name -match "\d$"}
dir c:\windows | where {$_.name -match "\s"}
dir HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\kb[0-9]*

I can't guarantee you'll get something for every single command, especially the registry example. But I hope you take the time to try them out. Regular expressions are extremely valuable in PowerShell and like any language element, practice makes perfect.

If you haven't already done so, be sure to read the help topic About_Regular_Expressions.