What characters are permitted for Haskell operators?

From the Haskell 2010 Report §2.4:

Operator symbols are formed from one or more symbol characters...

§2.2 defines symbol characters as being any of !#$%&*+./<=>?@\^|-~: or "any [non-ascii] Unicode symbol or punctuation".

NOTE: User-defined operators cannot begin with a : as, quoting the language report, "An operator symbol starting with a colon is a constructor."

From the Haskell report, this is the syntax for allowed symbols:

a | b means a or b and

a<b> means a except b

special    ->   ( | ) | , | ; | [ | ] | `| { | } 
symbol     ->   ascSymbol | uniSymbol<special | _ | : | " | '>
ascSymbol  ->   ! | # | $ | % | & | * | + | . | / | < | = | > | ? | @
                \ | ^ | | | - | ~
uniSymbol  ->   any Unicode symbol or punctuation 

So, symbols are ASCII symbols or Unicode symbols except from those in special | _ | : | " | ', which are reserved.

Meaning the following characters can't be used: ( ) | , ; [ ] ` { } _ : " '

A few paragraphs below, the report gives the complete definition for Haskell operators:

varsym     -> ( symbol {symbol | :})<reservedop | dashes>
consym     -> (: {symbol | :})<reservedop>
reservedop -> .. | : | :: | = | \ | | | <- | -> | @ | ~ | =>

Operator symbols are formed from one or more symbol characters, as defined above, and are lexically distinguished into two namespaces (Section 1.4):

• An operator symbol starting with a colon is a constructor.
• An operator symbol starting with any other character is an ordinary identifier.

Notice that a colon by itself, ":", is reserved solely for use as the Haskell list constructor; this makes its treatment uniform with other parts of list syntax, such as "[]" and "[a,b]".

Other than the special syntax for prefix negation, all operators are infix, although each infix operator can be used in a section to yield partially applied operators (see Section 3.5). All of the standard infix operators are just predefined symbols and may be rebound.