How can I filter filename patterns in PowerShell?

Question

I need to do something similar to Unix's ls | grep 'my[rR]egexp?' in Powershell. The similar expression ls | Select-String -Pattern 'my[rR]egexp?' seems to go through contents of the listed files, rather than simply filtering the filenames themselves.

The Select-String documentation hasn't been of much help either.

Note that Select-String patterns are case-insensitive by default. — Mike Zboray
– Mike Zboray, Commented Jun 9, 2015 at 18:54
@mikez: Not only Select-String. All of PowerShell (except when calling .NET methods directly). — Joey
– Joey, Commented Jun 11, 2015 at 10:37

Joey · Accepted Answer · 2015-06-09 18:34:48Z

6

Very simple:

ls | where Name -match 'myregex'

There are other options, though:

(ls) -match 'myregex'

Or, depending on how complex your regex is, you could maybe also solve it with a simple wildcard match:

ls wild[ck]ard*.txt

which is faster than above options. And if you can get it into a wildcard match without character classes you can also just use the -Filter parameter to Get-ChildItem (ls), which performs filtering on the file system level and thus is even faster. Note also that PowerShell is case-insensitive by default, so a character class like [rR] is unnecessary.

edited Jun 9, 2015 at 18:34

answered Jun 9, 2015 at 18:28

Joey

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3 Comments

Sundar R Over a year ago

Thanks, that's educative. How does the second method work though, are the parantheses a shortcut for some command? (btw the [rR] was just to illustrate it's a regex, my actual filter is based on \d and anchors)

TessellatingHeckler Over a year ago

The second method, the parentheses have the common programming/math "do this first" meaning; without them it would try and give -match as a parameter to ls but with them, it lists the files then does -match against the array of results.

Joey Over a year ago

The second one exploits a feature of PowerShell where comparison operators return matching results when there's a collection on the left side. (ls) first evaluates completely, yielding an array of results, -match is then applied for every one of them, trying to match the string representation (the name) to the regex, and returning those that match.

Sundar R · Accepted Answer · 2015-06-09 18:50:39Z

1

While researching based on @Joey's answer, I stumbled upon another way to achieve the same (based on Select-String itself):

ls -Name | Select-String -Pattern 'my[Rr]egexp?'

The -Name argument seems to make ls return the result as a plain string rather than FileInfo object, so Select-String treats it as the string to be searched in rather than a list of files to be searched.

answered Jun 9, 2015 at 18:50

Sundar R

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2 Comments

Joey Over a year ago

I'd really advise against this, though. PowerShell is designed to work with objects and while strings are objects, too, they're a poor substitute for richer ones, like FileInfo. Sure, you can go back to the Dark Age of Unix shell scripting, but you're missing out on a lot.

Sundar R Over a year ago

Haha yes, that was just my inner Unix geek finding comfort in a familiar plaintext pipe. Powershell's object functionalities do sound potentially much more powerful though, especially considering .NET is only a bracket away!

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