minibuffer-default-in-prompt-regexps

minibuffer-default-in-prompt-regexps is a variable defined in `minibuf-eldef.el'.
Its value is

(("\\( (default\\(?: is\\)? \\(.*\\))\\):? \\'" 1 nil)
 ("([^(]+?\\(, default\\(?: is\\)? \\(.*\\)\\)):? \\'" 1)
 ("\\( \\[.*\\]\\):? *\\'" 1))

Documentation:
A list of regexps matching the parts of minibuffer prompts showing defaults.
When `minibuffer-electric-default-mode' is active, these regexps are
used to identify the portions of prompts to elide.

Each entry is of the form (REGEXP MATCH-NUM &optional REWRITE),
where REGEXP should match the default part of the prompt,
MATCH-NUM is the subgroup that matched the actual default indicator,
and REWRITE, if present, is a string to pass to `replace-match' that
should be displayed in its place.