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about_Parsing

Updated: August 29, 2012

Applies To: Windows PowerShell 2.0, Windows PowerShell 3.0

TOPIC
    about_Parsing

SHORT DESCRIPTION
    Describes how Windows PowerShell parses commands.

LONG DESCRIPTION
    When you enter a command at the command prompt, Windows PowerShell
    breaks the command text into a series of segments called "tokens"
    and then determines how to interpret each "token." 

    For example, if you type:

         Write-Host book

    Windows PowerShell breaks the following command into two tokens, 
    "Write-Host" and "book", and interprets each token independently.

    When processing a command, the Windows PowerShell parser operates
    in expression mode or in argument mode: 

        - In expression mode, character string values must be contained in
          quotation marks. Numbers not enclosed in quotation marks are treated
          as numerical values (rather than as a series of characters). 

        - In argument mode, each value is treated as an expandable string 
          unless it begins with one of the following special characters: dollar
          sign ($), at sign (@), single quotation mark ('), double quotation
          mark ("), or an opening parenthesis (().


    If preceded by one of these characters, the value is treated as a value
    expression.

    The following table provides several examples of commands processed in 
    expression mode and argument mode and the results produced by those 
    commands.


    Example            Mode         Result
    ------------------ ----------   ----------------
    2+2                Expression   4 (integer)
    Write-Output 2+2   Argument     "2+2" (string)
    Write-Output (2+2) Expression   4 (integer)
    $a = 2+2           Expression   $a = 4 (integer)
    Write-Output $a    Expression   4 (integer)
    Write-Output $a/H  Argument     "4/H" (string)

    
    Every token can be interpreted as some kind of object type, such
    as Boolean or string. Windows PowerShell attempts to determine the
    object type from the expression. The object type depends on the
    type of parameter a command expects and on whether Windows PowerShell
    knows how to convert the argument to the correct type. The
    following table shows several examples of the types assigned to
    values returned by the expressions.


    Example            Mode         Result
    ------------------ ----------   ---------------
    Write-Output !1    argument     "!1" (string)
    Write-Output (!1)  expression   False (Boolean)
    Write-Output (2)   expression   2 (integer)

 STOP PARSING:  --%
    The stop-parsing symbol (--%), introduced in Windows PowerShell 3.0,
    directs Windows PowerShell to refrain from interpreting input as 
    Windows PowerShell commands or expressions.

    When calling an executable program in Windows PowerShell, place the 
    stop-parsing symbol before the program arguments. This technique is 
    much easier than using escape characters to prevent misinterpretation.  

   When it encounters a stop-parsing symbol, Windows PowerShell treats
   the remaining characters in the line as a literal. The only 
   interpretation it performs is to substitute values for environment 
   variables that use standard Windows notation, such as %USERPROFILE%. 

   The stop-parsing symbol is effective only until the next newline or 
   pipeline character. You cannot use a continuation character (`) to 
   extend its effect or use a command delimiter (;) to terminate its effect.

    For example, the following command calls the Icacls program.
        icacls X:\VMS /grant Dom\HVAdmin:(CI)(OI)F
    
    To run this command in Windows PowerShell 2.0, you must 
    use escape characters to prevent Windows PowerShell from
    misinterpreting the parentheses.

        icacls X:\VMS /grant Dom\HVAdmin:`(CI`)`(OI`)F

    Beginning in Windows PowerShell 3.0, you can use the stop-parsing
    symbol. 

        icacls X:\VMS --% /grant Dom\HVAdmin:(CI)(OI)F

    Windows PowerShell sends the following command string to the 
    Icacls  program: 
        X:\VMS /grant Dom\HVAdmin:(CI)(OI)F

SEE ALSO
    about_Command_Syntax



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