2 added 9 characters in body
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See the documentation for Optional:

The special form s_.s_. is equivalent to Optional[s_]Optional[s_] and can be used to represent function arguments which, if omitted, should be replaced by default values globally specified for the functions in which they occur.

This explains the result your are receiving.

Thus, it seems you have a precedence problem, and a possible workaround is:

Hold[(a_).(b_)] // FullForm

and get

Hold[Dot[Pattern[a, Blank[]], Pattern[b, Blank[]]]

Hold[Dot[Pattern[a, Blank[]], Pattern[b, Blank[]]]

See the documentation for Optional:

The special form s_. is equivalent to Optional[s_] and can be used to represent function arguments which, if omitted, should be replaced by default values globally specified for the functions in which they occur.

This explains the result your are receiving.

Thus, it seems you have a precedence problem, and a possible workaround is:

Hold[(a_).(b_)] // FullForm

and get

Hold[Dot[Pattern[a, Blank[]], Pattern[b, Blank[]]]

See the documentation for Optional:

The special form s_. is equivalent to Optional[s_] and can be used to represent function arguments which, if omitted, should be replaced by default values globally specified for the functions in which they occur.

This explains the result your are receiving.

Thus, it seems you have a precedence problem, and a possible workaround is:

Hold[(a_).(b_)] // FullForm

and get

Hold[Dot[Pattern[a, Blank[]], Pattern[b, Blank[]]]
1
source | link

See the documentation for Optional:

The special form s_. is equivalent to Optional[s_] and can be used to represent function arguments which, if omitted, should be replaced by default values globally specified for the functions in which they occur.

This explains the result your are receiving.

Thus, it seems you have a precedence problem, and a possible workaround is:

Hold[(a_).(b_)] // FullForm

and get

Hold[Dot[Pattern[a, Blank[]], Pattern[b, Blank[]]]