Searching for String Properties with Powershell

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I had a requirement recently to parse a configuration file (let’s just say for documentation purposes) and I needed to retrieve a property/value pair which may or may not be present in a text line. Now, depending on the product we wish to document, we might have a line in the configuration file constructed as follows:

set interface 1/3 -ifAlias DMZ -throughput 0 -bandwidthHigh 0 -bandwidthNormal 0 -intftype "Xen Virtual" -ifnum 1/3

Now, if wanted the “-intfType” value we could split the string using the Powershell .Split() method like so:

PS C:\> $SourceString = 'set interface 1/3 -ifAlias DMZ -throughput 0 -bandwidthHigh 0 -bandwidthNormal 0 -intftype "Xen Virtual" -ifnum 1/3';
$LineComponents = $SourceString.Split();
$LineComponents[12];
"Xen

However, there are two issues here:

  1. What happens if there are additional or missing properties (the index/order will change)?
  2. We were hoping/expecting to see “Xen Virtual” output and not just “Xen.

Here’s a quick function that will solve issue #1 (complete with case insensitivity):

#############################################################################
#.SYNOPSIS
# Get named property from a string.
#
#.DESCRIPTION
# Returns a case-insensitive property from a string, assuming the property is
# named before the actual property value and is separated by a space. For
# example, if the specified SearchString contained
# "-property1 <value1> -property2 <value2>”, searching # for "-Property1"
# would return "<value1>".
#
#.PARAMETER SearchString
# String to search for the specified property in.
#
#.PARAMETER PropertyName
# The property name to search for.
#
#.PARAMETER Default
# If the property is not found return the specified string. This parameter is
# optional and if not specified returns $null by default.
#
#.EXAMPLE
# $propertyValue = Get-StringProperty $StringToSearch "-property1"
#
#.EXAMPLE
# $propertyValue = Get-StringProperty $StringToSearch "-property3" "Not found"
##############################################################################
function Get-StringProperty([string]$SearchString, [string]$PropertyName, [string]$Default = $null)
{
    # Split the $SearchString based on one or more blank spaces
    $stringComponents = $SearchString.Split(' +',[StringSplitOptions]'RemoveEmptyEntries'); 
    for ($i = 0; $i -le $stringComponents.Length; $i++) {
        # The standard Powershell CompareTo method is case-sensitive
        if ([string]::Compare($stringComponents[$i], $PropertyName, $true) -eq 0) {
            # Check that we're not over the array boundary
            if ($i+1 -le $stringComponents.Length) {
                # If you wanted to trim quotation marks you could use this instead:
                #  return $StringComponents[$i+1].Trim('"');
                return $stringComponents[$i+1];
            }
        }
    }
    # If nothing has been found or we're over the array boundary, return the $EmptyString value
    return $Default;
}

$SourceString = 'set interface 1/3 -ifAlias DMZ -throughput 0 -bandwidthHigh 0 -bandwidthNormal 0 -intftype "Xen Virtual" -ifnum 1/3';
Get-StringProperty $SourceString "-intftype"

"Xen

Solving issue #2 is a little more involved as we need find the quoted text, escape it and then restore it when needed. The following Get-StringProperty advanced function will first replace all quoted spaces with “^” and then replace the quotes themselves with “^^”. This permits the split function to work as expected. Finally, everything is put back together just before we need it!

<#
.SYNOPSIS
   Get a named property value from a string.
.DESCRIPTION
   Returns a case-insensitive property from a string, assuming the property is
   named before the actual property value and is separated by a space. For
   example, if the specified SearchString contained "-property1 <value1>
   -property2 <value2>”, searching for "-Property1" would return "<value1>".
.PARAMETER SearchString
   String to search for the specified property name.
.PARAMETER PropertyName
   The property name to search the SearchString for.
.PARAMETER Default
   If the property is not found returns the specified string. This parameter is
   optional and if not specified returns $null (by default) if the property is
   not found.
.EXAMPLE
   Get-StringProperty -SearchString $StringToSearch -PropertyName "-property1"

   This command searches the $StringToSearch variable for the presence of the property
   "-property1" and returns its value, if found. If the property name is not found,
   the default $null will be returned.
.EXAMPLE
   Get-StringProperty $StringToSearch "-property3" "Not found"

   This command searches the $StringToSearch variable for the presence of the property
   "-property3" and returns its value, if found. If the property name is not found,
   the "Not Found" string will be returned.
.NOTES
   Author - Iain Brighton - @iainbrighton, iain.brighton@virtualengine.co.uk
#>
function Get-StringProperty {

    [CmdletBinding(HelpUri = 'https://virtualengine.co.uk/2014/searching-for-string-properties-with-powershell/')]
    [OutputType([String])]
    Param
    (
        [Parameter(Mandatory=$true, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true, Position=0)]
        [ValidateNotNullOrEmpty()]
        [Alias("Search")]
        [string]
        $SearchString,

        [Parameter(Mandatory=$true, ValueFromPipelineByPropertyName=$true, Position=1)]
        [ValidateNotNullOrEmpty()]
        [Alias("Name","Property")]
        [string]
        $PropertyName,

        [Parameter(ValueFromPipelineByPropertyName=$true, Position=2)]
        [AllowNull()]
        [String]
        $Default = $null
    )

    Begin { }

    Process {
        # Locate and replace quotes with '^^' and quoted spaces '^' to aid with parsing, until there are none left
        while ($SearchString.Contains('"')) {
            # Store the right-hand side temporarily, skipping the first quote
            $searchStringRight = $SearchString.Substring($SearchString.IndexOf('"') +1);
            # Extract the quoted text from the original string
            $quotedString = $SearchString.Substring($SearchString.IndexOf('"'), $searchStringRight.IndexOf('"') +2);
            # Replace the quoted text, replacing spaces with '^' and quotes with '^^'
            $SearchString = $SearchString.Replace($quotedString, $quotedString.Replace(" ", "^").Replace('"', "^^"));
        }

        # Split the $SearchString based on one or more blank spaces
        $stringComponents = $SearchString.Split(' +',[StringSplitOptions]'RemoveEmptyEntries'); 
        for ($i = 0; $i -le $stringComponents.Length; $i++) {
            # The standard Powershell CompareTo method is case-sensitive
            if ([string]::Compare($stringComponents[$i], $PropertyName, $True) -eq 0) {
                # Check that we're not over the array boundary
                if ($i+1 -le $stringComponents.Length) {
                    # Restore any escaped quotation marks and spaces
                    # If you wanted to trim quotation marks you could use this instead:
                    #  return $stringComponents[$i+1].Replace("^^", '"').Replace("^", " ").Trim('"');
                    return $stringComponents[$i+1].Replace("^^", '"').Replace("^", " ");
                }
            }
        }
        # If nothing has been found or we're over the array boundary, return the default value
        return $Default;
    }

    End { }
}

$SourceString = 'set interface 1/3 -ifAlias DMZ -throughput 0 -bandwidthHigh 0 -bandwidthNormal 0 -intftype "Xen Virtual" -ifnum 1/3';
Get-StringProperty $SourceString "-intftype"

"Xen Virtual"

That’s much better Smile. Just for good luck I have also created a complimentary Test-StringProperty advanced function that tests whether a property name is present or not. This removes a lot of if ((Get-StringPropertyValue $SearchString “PropertyName”) -ne $null) { Do-Something } calls.

<#
.SYNOPSIS
   Test for a named property value in a string.
.DESCRIPTION
   Tests for the presence of a property value in a string and returns a boolean
   value. For example, if the specified SearchString contained "-property1
   -property2 <value2>”, searching for "-Property1" or "-Property2" would return
   $true, but searching for "-Property3" would return $false
.PARAMETER SearchString
   String to search for the specified property name.
.PARAMETER PropertyName
   The property name to search the SearchString for.
.EXAMPLE
   Test-StringProperty -SearchString $StringToSearch -PropertyName "-property1"

   This command searches the $StringToSearch variable for the presence of the property
   "-property1". If the property name is found it returns $true. If the property name
   is not found, it will return $false.
.NOTES
   Author - Iain Brighton - @iainbrighton, iain.brighton@virtualengine.co.uk
#>
function Test-StringProperty {

    [CmdletBinding(HelpUri = 'https://virtualengine.co.uk/2014/searching-for-string-properties-with-powershell/')]
    [OutputType([bool])]
    Param
    (
        [Parameter(Mandatory=$true, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true, Position=0)]
        [ValidateNotNullOrEmpty()]
        [Alias("Search")]
        [string]
        $SearchString,

        [Parameter(Mandatory=$true, ValueFromPipelineByPropertyName=$true, Position=1)]
        [ValidateNotNullOrEmpty()]
        [Alias("Name","Property")]
        [string]
        $PropertyName
    )

    Begin { }

    Process {
        # Split the $SearchString based on one or more blank spaces
        $stringComponents = $SearchString.Split(' +',[StringSplitOptions]'RemoveEmptyEntries'); 
        for ($i = 0; $i -le $stringComponents.Length; $i++) {
            # The standard Powershell CompareTo method is case-sensitive
            if ([string]::Compare($stringComponents[$i], $PropertyName, $True) -eq 0) { return $true; }
        }
        # If nothing has been found or we're over the array boundary, return the default value
        return $false;
    }

    End { }
}

Some final words of warning:

  • If there are escaped (double) quotes then this function won’t work without some modification.
  • If the $SearchString just so happens to natively contain either “^” and/or “^^” it won’t (currently) work either.
  • If you have any improvements or feedback then please let me know.
  • Please test in your environment before putting any 3rd party or external code into production!
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