Bash Parameter Expansion says that the variable (FILE in your example) must be a parameter name. So they don't nest. And the last part of ${param/pattern/replacement} must be a string. So back references aren't supported.

My only advice is to use

${EXT:+.$EXT}

to avoid adding a trailing dot if the file has no extension.

(Apparently back references may be supported in ksh93)

Bash Parameter Expansion says that the variable (FILE in your example) must be a parameter name. So they don't nest. And the last part of ${param/pattern/replacement} must be a string. So back references aren't supported.

My only advice is to use

${EXT:+.$EXT}

to avoid adding a trailing dot if the file has no extension.

UPDATE

Apparently back references are supported in ksh93.

So you could use something like

FILE="${FILE/@(.*)/_something\1}"

Bash Parameter Expansion says that the variable (FILE in your example) must be a parameter name. So they don't nest. And the last part of ${param/pattern/replacement} must be a string. So back references aren't supported.

My only advice is to use

${EXT:+.$EXT}

to avoid adding a trailing dot if the file has no extension.

Bash Parameter Expansion says that the variable (FILE in your example) must be a parameter name. So they don't nest. And the last part of ${param/pattern/replacement} must be a string. So back references aren't supported.

My only advice is to use

${EXT:+.$EXT}

to avoid adding a trailing dot if the file has no extension.

(Apparently back references may be supported in ksh93)