Complicated!

pattern	interpolated	repeat expression
$x{digits[,[digits]]}	$x	{digits[,[digits]]}
$x{y}	$x{y}	once
${x}{digits[,[digits]]}	$x	{digits[,[digits]]}
${x}{y}	$x	once (The rest will be treated independently)
${x{y}}{digits[,[digits]]}	$x{y}	{digits[,[digits]]}
${x{y}}{z}	$x{y}	once (The rest will be treated independently)

The rules seem to be

• {...} is always treated as a repeat expression if it resembles a repeat expression.
• {...} is never treated as a repeat expression if it doesn't resembles a repeat expression.
• ${...} is always treated as a complete variable interpolation.

But it's completely different for [...].

pattern	interpolated	rest
$x[y]	$x[y]
${x}[y]	$x	[y]

The rules seem to be

• [...] is always treated as an array index if an array index is acceptable.
• [...] is never treated as an array index if an array index isn't acceptable.
• ${...} is always treated as a complete variable interpolation.