PowerShell string interpolation syntax

To complement marsze's helpful answer:

${...} (enclosing the variable name in { and }) is indeed always necessary if a variable name contains special characters, such as spaces, ., or -.

• Not special are _ and - surprisingly and problematically - ?.
• Note: : is invariably interpreted as terminating a PowerShell drive reference, in the context of namespace variable notation, irrespective of whether {...} enclosure is used or required (e.g., in $env:USERNAME or ${env:USERNAME}, env refers to the PowerShell drive representing all environment variables).

In the context of string expansion (interpolation) inside "...", there is another reason to use ${...}, even if the variable name itself doesn't need it:

If you need to delineate the variable name from directly following non-whitespace characters, notably including ::

$foo = 'bar'  # example variable# INCORRECT: PowerShell assumes that the variable name is 'foobarian', not 'foo'PS> "A $foobarian."A .  # Variable $foobarian doesn't exist -> reference expanded to empty string.# CORRECT: Use {...} to delineate the variable name:PS> "A ${foo}barian."A barbarian.# INCORRECT: PowerShell assumes that 'foo:' is a *namespace* (drive) reference#            (such as 'env:' in $env:PATH) and FAILS:PS> "$foo: bar"Variable reference is not valid. ':' was not followed by a valid variable name character. Consider using ${} to delimit the name.# CORRECT: Use {...} to delineate the variable name:PS> "${foo}: bar"bar: bar

See this answer for a comprehensive overview of PowerShell string-expansion rules.

Note that you need the same technique when string expansion is implicitly applied, in the context of passing an unquoted argument to a command; e.g.:

# INCORRECT: The argument is treated as if it were enclosed in "...",#            so the same rules apply.Write-Output $foo:/bar# CORRECTWrite-Output ${foo}:/bar

Finally, a somewhat obscure alternative is to `-escape the first character after the variable name, but the problem is that this only works as expected with characters that aren't part of escape sequences (see about_Special_Characters):

# OK: because `: is not an escape sequence.PS> "$foo`: bar"bar: bar# NOT OK, because `b is the escape sequence for a backspace character.PS> "$foo`bar"baar # The `b "ate" the trailing 'r' of the variable value     # and only "ar" was the literal part.

${variable} is the syntax for variable names that include special characters.

(See about_Variables -> Variable names that include special characters)

Example:

${var with spaces} = "value""var with spaces: ${var with spaces}"

So in your case it's basically the same as simply writing $variable