[antlr-interest] Can Antlr use a variable in a lexer pattern?

[Douglas Godfrey]

in the snippet below, can "escape_character" be a variable?
it seems that this would not work because the "escape_character" is not
known until it is too late.
the alternate form below might work if the Antlr Lexer can use a variable in
the pattern match.
can the lexer apply the escape character as a post processing validation
step?
    i.e. accept anything within the quotes and then validate the sequence
after the ESCAPE clause?

Unicode_Identifier  =
        U Ampersand
        Double_Quote  ( Unicode_Identifier_Part )+ Double_Quote
        ( ESCAPE escape_character )?
        ;


Alternate form:

Unicode_Identifier  =
        U Ampersand
        ( ESCAPE escape_character )?
        Double_Quote  ( Unicode_Identifier_Part )+ Double_Quote
        ;


fragment
Unicode_Identifier_Part  = Unicode_Permitted_Identifier_Character  |
Unicode_Escape_Value ;

fragment
Unicode_Escape_Value  = Unicode_4_Digit_Escape_Value  |
Unicode_6_Digit_Escape_Value ;

fragment
Unicode_4_Digit_Escape_Value  = escape_character  Hexit  Hexit  Hexit  Hexit
;

fragment
Unicode_6_Digit_Escape_Value  = escape_character  Plus_Sign Hexit  Hexit
Hexit  Hexit  Hexit  Hexit ;

escape_character            = Back_Slash /*!! See the Syntax Rules*/; ;