About Parsing
SHORT DESCRIPTION
Describes how PowerShell parses commands.
LONG DESCRIPTION
When you enter a command at the command prompt, 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
PowerShell breaks the following command into two tokens, "Write-Host" and "book", and interprets each token independently.
When processing a command, the 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. 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 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) |
The stop-parsing symbol (--%), introduced in PowerShell 3.0, directs PowerShell to refrain from interpreting input as PowerShell commands or expressions.
When calling an executable program in 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, 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 PowerShell 2.0, you must use escape characters to prevent PowerShell from misinterpreting the parentheses.
icacls X:\VMS /grant Dom\HVAdmin:`(CI`)`(OI`)F
Beginning in PowerShell 3.0, you can use the stop-parsing symbol.
icacls X:\VMS --% /grant Dom\HVAdmin:(CI)(OI)F
PowerShell sends the following command string to the Icacls program:
X:\VMS /grant Dom\HVAdmin:(CI)(OI)F