Powershell - Single Quote in String Variable with XPath

Question

Dear Powershell fellows,

i have a (maybe) very simple problem, but i have no idea how to solve it. I want to use a String Variable, that contains a single quote, within a XPath notation. I use the CMDlet Select-Xml. If there is no single quote within the String, Select-Xml is working completly fine. But if there is one single quote (for example in: don't) it crashes my script. Let me show you in detail.

Problem

$Name = "Dont display" ##is working completly fine

(Select-Xml -Path "C:\SomePath\Somexml.xml" -XPath "//q1:Options[q1:Name = '$Name']"  -Namespace $namespace).Node.InnerText  ##is working completly fine

$Name = "Don't display" ##crashes script

(Select-Xml -Path "C:\SomePath\Somexml.xml" -XPath "//q1:Options[q1:Name = '$Name']"  -Namespace $namespace).Node.InnerText  ##crashes script

The error output of powershell is:

 Select-Xml : '//q1:Options[q1:Name = 'Don't display']' has an invalid token.
    At line:251 char:41
    + ... ing_Name = (Select-Xml -Path "C:\SomePath\Somexml.xml" -XPath ...
    +                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        + CategoryInfo          : NotSpecified: (:) [Select-Xml], XPathException
        + FullyQualifiedErrorId : System.Xml.XPath.XPathException,Microsoft.PowerShell.Commands.SelectXmlCommand

What i tried so far

Of course i tried different quotations, such as:

 $Name = """Don't display"""
 $Name = '"Don't display"'
 $Name = "'Don't display'"
 $Name = "'Don't display'"
 $Name = 'Don't display'
$Name = ''Don't display''

It seems like, there might be a problem with powershell quotation rules and XPath notation.

But maybe anyone of you guys have an idea on how to solve it.

Thank you very much

Ideally your XPath API would provide you with a way to pass in a variable value so that your XPath would be e.g. `//q1:Options[q1:Name = $dp]` where you could pass in a string value "Don't display" for the variable `dp`. But I don't think the Powershell or .NET XPath API has an easy interface for XPath variables. — Martin Honnen, Sep 01 '22 at 08:52
Maybe try escaping the apostrophe with `$Name = "Don't display" -replace "'", '\"''\"'` ? (results in a string `Don\"'\"t display`) — Theo, Sep 01 '22 at 09:13

mclayton · Accepted Answer · 2022-09-01T22:04:52.550

It seems there's no escape character in XPath string literals, so you can't use your string delimiter inside a literal string as it terminates the string - i.e. this:

$name = "Don't display"

# single quotes:
# //q1:Options[q1:Name = 'Don't display']
#                            ^ terminates string literal

A quick (but naive) solution to your specific issue would be to just use double-quotes as delimiters instead:

$name = "Don't display"

# double quotes:
# //q1:Options[q1:Name = "Don't display"]
#                            ^ *doesn't* terminate string literal

but what if your data contains double quotes? Then you're back at square one..

$name = "Don""t display"

# double quotes:
# //q1:Options[q1:Name = "Don"t display"]
#                            ^ terminates string literal again

And in a pathological case, if your literal contains both single and double quotes then you can't use either as delimiters:

$name = "Don't d""isplay"

# single quotes:
# //q1:Options[q1:Name = 'Don't d"isplay']
#                            ^ terminates string literal

# double quotes:
# //q1:Options[q1:Name = "Don't d"isplay"]
#                                ^ also terminates string literal

In that case, you could resort to this answer which suggests converting your string literal into a concat expression so that you get:

$name = "Don't d""isplay"

# //q1:Options[q1:Name = concat('Don', "'", 't d"isplay')]
#                               ^^^^^ use single quotes
#                                      ^^^ use double quotes
#                                           ^^^^^^^^^^^^ use single quotes

which you could generate with this:

$name = "Don't d""isplay"

$squote = "', `"'`", '"
$expr   = "concat('{0}')" -f $name.Replace("'", $squote)

# Select-Xml -Xml $xml -XPath "//q1:Options[q1:Name = $expr]"
# ->
# //q1:Options[q1:Name = concat('Don', "'", 't d"isplay')]

and then the parts of your data that contain double-quotes are delimited with single quotes, and vice versa so they all terminate properly.

Note - you could probably optimise this for literals without one type of quote or the other and for consecutive single quotes, and it'll need some error handling added for $null and other edge cases, but it basically does the job...

Update

Here's a full code sample to show it in action...

$xml = [xml] "<root><child Name=`"my'name`" /></root>"

$name = "my'name"

$squote = "', `"'`", '"
$expr   = "concat('{0}')" -f $name.Replace("'", $squote)

Select-Xml -Xml $xml -XPath "//child[@Name = $expr]"

# Node  Path        Pattern
# ----  ----        -------
# child InputStream //child[@Name = concat('my', "'", 'name')]

Thank you very much. This is a really strong answer and the solution with the $expr solved error message "terminates string literal". But unfortunately now my Select-Xml gives me back a null. Dont know why exactly yet, maybe Select-XML cant handle the concat input. I will investigate this problem further. Thank you for help ! — LKo.exp, Sep 01 '22 at 12:16
@LKo.exp - I've added a stand-alone example to show it working with ```Select-Xml``` - you might need to check your xpath is correct, but ```concat``` seems to work fine if it actually matches something (at least in PS 7.2.6...) — mclayton, Sep 01 '22 at 12:42
Thank you very much!!! Found the additional mistake in my script and now it is working like a charme. :) — LKo.exp, Sep 01 '22 at 12:59

Powershell - Single Quote in String Variable with XPath

1 Answers1