Question

Let’s say I have the results of a call to Select-String in a variable $mat, parsing a regular expression from file contents:

$mat = cat errors.txt | Select-String "'(?<code>\w+)'.+ID (?<id>[^:]+)" 

According to the output of $mat | Get-Member the result contains a Matches property of type Match[].

When I execute the following I get all the matches of my regular expression output:

PS > $mat | Select-Object -Property Matches

Matches                                                                                                                                                                              
-------                                                                                                                                                                              
{'ACCFWD', ID 16}                                                                                                                                                                    
{'EQASIAN', ID 448}                   
    

Why doesn’t this next block of code using foreach to select the Matches have the same output:

    PS > $mat | ForEach { $_.Matches } 


Groups   : {'ACCFWD', ID 16, ACCFWD, 16}
Success  : True
Captures : {'ACCFWD', ID 16}
Index    : 20
Length   : 15
Value    : 'ACCFWD', ID 16

Groups   : {'EQASIAN', ID 448, EQASIAN, 448}
Success  : True
Captures : {'EQASIAN', ID 448}
Index    : 20
Length   : 17
Value    : 'EQASIAN', ID 448

Answer 1

When displaying properties, PowerShell auto-formats the properties of types that do not have a display format defined in a *.format.ps1xml file as a table for up to 4 properties. 5 or more properties displays as a list. When you select the Matches property with Select-Object, you are selecting a single property of the Microsoft.PowerShell.Commands.MatchInfo object. With Foreach-Object you are displaying all of the properties for a System.Text.RegularExpressions.Match object.

Using Select-Object -ExpandProperty Matches will cause the output to look the same as the Foreach because it will output RegularExpressions.Match objects.

If you place Get-Member after both of your examples that produce output you will see that they output different types of objects.

Edit: Here is an explanation of the formatting that happens for each command.

cat errors.txt | Select-String "'(?<code>\w+)'.+ID (?<id>[^:]+)"

The output of Select-String is a Microsoft.PowerShell.Commands.MatchInfo object which has 8 properties. These properties are not displayed by default because the display format for MatchInfo is defined in PowerShellCore.format.ps1xml to show the result of MatchInfo‘s ToString() method.

$mat | Select-Object -Property Matches

In this case, the output of Select-Object is a custom Selected.Microsoft.PowerShell.Commands.MatchInfo object with the Matches property that was copied from the MatchInfo object. Since there is no default display format defined for the Selected.Microsoft.PowerShell.Commands.MatchInfo type, PowerShell auto formats it as a table since it has less than 5 properties (In this case Matches is the only property).

$mat | ForEach { $_.Matches } 

In the Foreach-Object ScriptBlock, the Matches property of the MatchInfo object is being output. The Matches property is a System.Text.RegularExpressions.Match which has 6 properties. Since there is no default display format defined for the System.Text.RegularExpressions.Match type, the Match objects are displayed as a list because there are more than 4 properties.