completion-pcm-word-delimiters

completion-pcm-word-delimiters is a variable defined in `minibuffer.el'.
Its value is

"-_./:| "

Documentation:
A string of characters treated as word delimiters for completion.
Some arcane rules:
If `]' is in this string, it must come first.
If `^' is in this string, it must not come first.
If `-' is in this string, it must come first or right after `]'.
In other words, if S is this string, then `[S]' must be a valid Emacs regular
expression (not containing character ranges like `a-z').

You can customize this variable.