non/grep.tal

## grep.tal
( grep.tal )
( )
( by d_m )

( print a character to STDOUT )
%emit { #18 DEO }

( the first argument to grep is the regex )
( arguments are passed on STDIN, so we just )
( assume the first "line" is the argument )
( and the rest are lines to be grepped )
|0100
    ;r-read-stdin #10 DEO2 BRK

@regex 0000   ( compiled regex address (if any) )
@buffer $1000 ( buffer to read user input )
@ptr :buffer  ( next byte to write in buffer )

@println ( s* -> )
    &loop LDAk #00 EQU ,&eof JCN ( did we reach \0 ? )
    LDAk emit INC2 ,&loop JMP    ( no so emit a char and repeat )
    &eof #0a emit POP2 JMP2r     ( yes so emit \n and return )

@r-read-stdin ( -> )
    #12 DEI #0a EQU ,&execute JCN     ( did we read \n ? )
    #12 DEI ;ptr LDA2 STA             ( no, so save in buffer )
    ;ptr LDA2k INC2 SWP2 STA2         ( ptr++ )
    BRK                               ( return )

    &execute                          ( we saw a newline, so do something )
    #00 ;ptr LDA2 STA                 ( null terminate str )
    ;buffer ;ptr STA2                 ( reset ptr )
    ;regex LDA2 #0000 EQU2            ( is regex initialized? )
    ,&need-regex JCN                  ( jump if unset )

                                      ( regex is set )
    ;buffer ;regex LDA2 ;match JSR2   ( match regex )
    #00 EQU ,&no-match JCN            ( did we match? )
    ;buffer ;println JSR2             ( print any match )
    &no-match BRK                     ( return )

                                      ( regex is unset )
    &need-regex ;buffer ;compile JSR2 ( compile regex )
    ;regex STA2 BRK                   ( save regex and return )

( include the actual regex machinery )
~regex.tal

## regex.tal
( regex.tal                                                        )
(                                                                  )
( compiles regex expression strings into regex nodes, then uses    )
( regex ndoes to match input strings.                              )
(                                                                  )
( this currently only supports matching an entire string, as       )
( opposed to searching for a matching substring, or extracting     )
( matching subgroups.                                              )
(                                                                  )
( regex node types:                                                )
(                                                                  )
(   NAME    DESCRIPTION                      STRUCT                )
(   empty   matches empty string             [ #01 next* ]         )
(   dot     matches any one char             [ #02 next* ]         )
(   lit     matches one specific char (c)    [ #03 c^ next* ]      )
(   or      matches either left or right     [ #04 left* right* ]  )
(   star    matches expr zero-or-more times  [ #05 r* next* ]      )
(           (NOTE: r.expr.next must be r)                          )
(                                                                  )
( `or` and `star` have the same structure and are handled by the   )
( same code (;do-or). however, the node types are kept different   )
( to make it clearer how to parse and assemble the nodes.          )
(                                                                  )
( concatenation isn't a node, it is implied by the *next addr.     )
( a next value of #0000 signals the end of the regex.              )
(                                                                  )
( in these docs str* is an address to a null-terminated string.    )
( regexes should not include nulls and cannot match them (other    )
( than the null which signals the end of a string).                )

%null? { #00 EQU }
%debug { #ff #0e DEO }
%emit { #18 DEO }
%space { #20 emit }
%newline { #0a emit }
%quit! { #01 #0f DEO BRK }

( ERROR HANDLING )

( using error! will print the given message before causing )
( the interpreter to halt. )
@error! ( msg* -> )
    LIT '! emit space
    &loop LDAk ,&continue JCN ,&done JMP
    &continue LDAk emit INC2 ,&loop JMP
    &done POP2 newline quit!

( error messages )
@unknown-node-type "unknown 20 "node 20 "type 00
@mismatched-parens "mismatched 20 "parenthesis 00
@stack-is-full "stack 20 "is 20 "full 00
@stack-is-empty "stack 20 "is 20 "empty 00
@arena-is-full "arena 20 "is 20 "full 00
@star-invariant "star 20 "invariant 20 "failed 00
@plus-invariant "plus 20 "invariant 20 "failed 00
@qmark-invariant "question 20 "mark 20 "invariant 20 "failed 00

( REGEX MATCHING )

( use stored regex to match against a stored string. )
( )
( regex* should be the address of a compiled regex )
( such as that returned from ;compile. )
( )
( str* should be a null-terminated string. )
( )
( returns true if the string, and false otherwise. )
@match ( str* regex* -> bool^ )
    ;reset-stack JSR2
    ;loop JMP2

( loop used during matching )
( )
( we don't use the return stack here since that )
( complicates the back-tracking we need to do. )
( ultimately this code will issue a JMP2r to )
( return a boolean, which is where the stack )
( effects signature comes from. )
@loop ( s* r* -> bool^ )
    LDAk #01 EQU ;do-empty JCN2
    LDAk #02 EQU ;do-dot JCN2
    LDAk #03 EQU ;do-literal JCN2
    LDAk #04 EQU ;do-or JCN2
    LDAk #05 EQU ;do-or JCN2 ( same code as the or case )
    ;unknown-node-type ;error! JSR2

( used when we hit a dead-end during matching. )
( )
( if stack is non-empty we have a point we can resume from. )
@goto-backtrack ( -> bool^ )
    ;stack-exist JSR2 ,&has-stack JCN ( do we have stack? )
    #00 JMP2r ( no, return false )
    &has-stack ;pop4 JSR2 ;goto-next JMP2 ( yes, resume from the top )

( follow the given address (next*) to continue matching )
@goto-next ( str* next* -> bool^ )
    DUP2 #0000 GTH2 ,&has-next JCN
    POP2 LDA null? ,&end-of-string JCN
    ;goto-backtrack JMP2
    &end-of-string #01 JMP2r
    &has-next ;loop JMP2

( handle the empty node -- just follow the next pointer )
@do-empty ( str* regex* -> bool^ )
    INC2 LDA2 ( load next )
    ;goto-next JMP2 ( jump to next )

( handle dot -- match any one character )
@do-dot ( str* regex* -> bool^ )
    INC2 LDA2 STH2 ( load and stash next )
    LDAk #00 NEQ ,&non-empty JCN ( is there a char? )
    POP2r POP2 ;goto-backtrack JMP2 ( no, clear stacks and backtrack )
    &non-empty INC2 STH2r ;goto-next JMP2 ( yes, inc s, restore and jump )

( handle literal -- match one specific character )
@do-literal ( str* regex* -> bool^ )
    INC2
    LDAk STH ( store c )
    INC2 LDA2 STH2 ROTr ( store next, move c to top )
    LDAk
    STHr EQU ,&matches JCN ( do we match this char? )
    POP2r POP2 ;goto-backtrack JMP2 ( no, clear stacks and backtrack )
    &matches
    INC2 STH2r ;goto-next JMP2 ( yes, inc s, restore and jump )

( handle or -- try the left branch but backtrack to the right if needed )
( )
( this also handles asteration, since it ends up having the same structure )
@do-or ( str* regex* -> bool^ )
    INC2 OVR2 OVR2 #0002 ADD2 ( s r+1 s r+3 )
    LDA2 ;push4 JSR2 ( save (s, right) in the stack for possible backtracking )
    LDA2 ;loop JMP2 ( continue on left branch )

( REGEX PARSING )

( track the position in the input string )
@pos $2

( track how many levels deep we are in parenthesis )
@parens $2

( read and increment pos )
@read ( -> c^ )
    ;pos LDA2k ( pos s )
    LDAk STHk #00 EQU ( pos s c=0 [c] )
    ,&is-eof JCN ( pos s [c] )
    INC2 ( pos s+1 [c] )
    SWP2 STA2 ,&return JMP ( [c] )
    &is-eof POP2 POP2
    &return STHr ( c )
    JMP2r

( is pos currently pointing to a star? )
@peek-to-star ( -> is-star^ )
    ;pos LDA2 LDA LIT '* EQU JMP2r

( is pos currently pointing to a plus? )
@peek-to-plus ( -> is-plus^ )
    ;pos LDA2 LDA LIT '+ EQU JMP2r

( is pos currently pointing to a qmark? )
@peek-to-qmark ( -> is-qmark^ )
    ;pos LDA2 LDA LIT '? EQU JMP2r

( just increment pos )
@skip
    ;pos LDA2 INC2 ;pos STA2 JMP2r

( TODO: )
( 1. character groups: [] and [^] )
( 2. symbolic escapes, e.g. \n )

( STRETCH GOALS: )
( a. ^ and $ )
( b. counts: {n} and {m,n} )
( c. substring matching, i.e. searching )
( d. subgroup extraction )
( e. back-references, e.g \1 )

( compile an expression string into a regex graph )
( )
( the regex will be allocated in the arena; if there is not )
( sufficient space an error will be thrown. )
( )
( the stack will also be used during parsing although unlike )
( the arena it will be released once compilation ends. )
@compile ( expr* -> regex* )
          ;pos    STA2
    #0000 ;parens STA2
    ;reset-stack JSR2
    ;compile-region JMP2

( the basic strategy here is to build a stack of non-or )
( expressions to be joined together at the end of the )
( region. each stack entry has two regex addresses: )
(   - the start of the regex )
(   - the current tail of the regex )
( when we concatenate a new node to a regex we update )
( the second of these but not the first. )
( )
( the bottom of the stack for a given region is denoted )
( by #ffff #ffff. above that we start with #0000 #0000 )
( to signal an empty node. )
@compile-region ( -> r2* )
    #ffff #ffff ;push4 JSR2 ( stack delimiter )
    #0000 #0000 ;push4 JSR2 ( stack frame start )
@compile-region-loop
    ;read JSR2
    DUP #00 EQU ;c-done JCN2
    DUP LIT '| EQU ;c-or   JCN2
    DUP LIT '. EQU ;c-dot  JCN2
    DUP LIT '( EQU ;c-lpar JCN2
    DUP LIT ') EQU ;c-rpar JCN2
    DUP LIT '\ EQU ;c-esc  JCN2
    DUP LIT '* EQU ;c-star JCN2
    DUP LIT '+ EQU ;c-plus JCN2
    DUP LIT '? EQU ;c-qmark JCN2
                   ;c-char JMP2

( either finalize the given r0/r1 or else wrap it in )
( a star node if a star is coming up next. )
( )
( we use this look-ahead approach rather than compiling )
( star nodes directly since the implementation is simpler. )
@c-peek-and-finalize ( r0* r1* -> r2* )
    ;peek-to-star JSR2 ( r0 r1 next-is-star? ) ,&next-is-star JCN
    ;peek-to-plus JSR2 ( r0 r1 next-is-plus? ) ,&next-is-plus JCN
    ;peek-to-qmark JSR2 ( r0 r1 next-is-qmark? ) ,&next-is-qmark JCN
    ,&finally JMP ( r0 r1 )
    &next-is-star ;skip JSR2 POP2 ;alloc-star JSR2 DUP2 ,&finally JMP
    &next-is-plus ;skip JSR2 POP2 ;alloc-plus JSR2 DUP2 ,&finally JMP
    &next-is-qmark ;skip JSR2 POP2 ;alloc-qmark JSR2 DUP2 ,&finally JMP
    &finally ;push-next JSR2 ;compile-region-loop JMP2

( called when we reach EOF of the input string )
( )
( as with c-rpar we have to unroll the current level )
( of the stack, building any or-nodes that are needed. )
( )
( this is where we detect unclosed parenthesis. )
@c-done ( c^ -> r2* )
    POP
    ;parens LDA2 #0000 GTH2 ,&mismatched-parens JCN
    ;unroll-stack JSR2 POP2 JMP2r
    &mismatched-parens ;mismatched-parens ;error! JSR2

( called when we read "|" )
( )
( since we defer building or-nodes until the end of the region )
( we just start a new stack frame and continue. )
@c-or ( c^ -> r2* )
    POP
    #0000 #0000 ;push4 JSR2
    ;compile-region-loop JMP2

( called when we read "(" )
( )
( this causes us to: )
( )
(  1. increment parens )
(  2. start a new region on the stack )
(  3. jump to compile-region to start parsing the new region )
@c-lpar ( c^ -> r2* )
    POP
    ;parens LDA2 INC2 ;parens STA2 ( parens++ )
    ;compile-region JMP2

( called when we read ")" )
( )
( this causes us to: )
( )
(  1. check for mismatched parens )
(  2. decrement parens )
(  3. unroll the current region on the stack into one regex node )
(  4. finalize that node and append it to the previous region )
(  5. continue parsing )
@c-rpar ( c^ -> r2* )
    POP
    ;parens LDA2 #0000 EQU2 ,&mismatched-parens JCN
    ;parens LDA2 #0001 SUB2 ;parens STA2 ( parens-- )
    ;unroll-stack JSR2
    ;c-peek-and-finalize JMP2
    &mismatched-parens ;mismatched-parens ;error! JSR2

( called when we read "." )
( )
( allocates a dot-node and continues. )
@c-dot ( c^ -> r2* )
    POP
    ;alloc-dot JSR2 ( dot )
    DUP2 ;c-peek-and-finalize JMP2

( TODO: escaping rules not quite right )

( called when we read "\" )
( )
( allocates a literal of the next character. )
( )
( this doesn't currently handle any special escape sequences. )
@c-esc ( c^ -> r2* )
    POP
    ;read JSR2
    ;c-char JMP2

( called when we read any other character )
( )
( allocates a literal-node and continues. )
@c-char ( c^ -> r2* )
    ;alloc-lit JSR2 ( lit )
    DUP2 ;c-peek-and-finalize JMP2

( called if we parse a "*" )
( )
( actually calling this means the code broke an invariant somewhere. )
@c-star ( c^ -> regex* )
    POP
    ;star-invariant ;error! JSR2

( called if we parse a "+" )
( )
( actually calling this means the code broke an invariant somewhere. )
@c-plus ( c^ -> regex* )
    POP
    ;plus-invariant ;error! JSR2

( called if we parse a "?" )
( )
( actually calling this means the code broke an invariant somewhere. )
@c-qmark ( c^ -> regex* )
    POP
    ;qmark-invariant ;error! JSR2

( ALLOCATING REGEX NDOES )

@alloc3 ( mode^ -> r* )
    #0000 ROT ( 00 00 mode^ )
    #03 ;alloc JSR2 ( 00 00 mode^ addr* )
    STH2k STA ( addr <- mode )
    STH2kr INC2 STA2 ( addr+1 <- 0000 )
    STH2r JMP2r ( return addr )

@alloc-empty ( -> r* )
    #01 ;alloc3 JMP2

@alloc-dot ( -> r* )
    #02 ;alloc3 JMP2

@alloc-lit ( c^ -> r* )
    #03 #0000 SWP2 ( 0000 c^ 03 )
    #04 ;alloc JSR2 ( 0000 c^ 03 addr* )
    STH2k STA ( addr <- 03 )
    STH2kr INC2 STA ( addr+1 <- c )
    STH2kr #0002 ADD2 STA2 ( addr+2 <- 0000 )
    STH2r JMP2r ( return addr )

@alloc-or ( right* left* -> r* )
    #05 ;alloc JSR2 STH2 ( r l [x] )
    #04 STH2kr            STA ( r l [x] )
        STH2kr       INC2 STA2 ( r [x] )
        STH2kr #0003 ADD2 STA2 ( [x] )
    STH2r JMP2r

@alloc-star ( expr* -> r* )
    #05 ;alloc JSR2 STH2  ( expr [r] )
    #05 STH2kr STA        ( expr [r] )
    DUP2 STH2kr INC2 STA2 ( expr [r] )
    #0000 STH2kr #0003 ADD2 STA2 ( expr [r] )
    STH2kr SWP2 ( r expr [r] )
    ;set-next JSR2 ( [r] )
    STH2r JMP2r

@alloc-plus ( expr* -> r* )
    #05 ;alloc JSR2 STH2  ( expr [r] )
    #05 STH2kr STA        ( expr [r] )
    DUP2 STH2kr INC2 STA2 ( expr [r] )
    #0000 STH2kr #0003 ADD2 STA2 ( expr [r] )
    STH2r SWP2 STH2k ( r expr [expr] )
    ;set-next JSR2 ( [expr] )
    STH2r JMP2r

@alloc-qmark ( expr* -> r* )
    ;alloc-empty JSR2 STH2k ( expr e [e] )
    OVR2 ;set-next JSR2 ( expr [e] )
    #05 ;alloc JSR2 STH2  ( expr [r e] )
    #04 STH2kr STA        ( expr [r e] )
    STH2kr INC2 STA2 ( [r e] )
    SWP2r STH2r STH2kr ( e r [r] )
    #0003 ADD2 STA2 ( [r] )
    STH2r JMP2r

( if r is 0000, allocate an empty node )
@alloc-if-null ( r* -> r2* )
    ORAk ,&return JCN POP2 ;alloc-empty JSR2 &return JMP2r

( unroll one region of the parsing stack, returning )
( a single node consisting of an alternation of )
( all elements on the stack. )
( )
( this unrolls until it hits #ffff #ffff, which it )
( also removes from the stack. )
@unroll-stack ( -> start* end* )
    ;pop4 JSR2 STH2 ( r )
    #00 STH ( count items in stack frame )
    ;alloc-if-null JSR2 ( replace 0000 with empty )
    &loop ( r* )
    ;pop4 JSR2 POP2 ( r x )
    DUP2 #ffff EQU2 ( r x x-is-end? ) ,&done JCN
    INCr ( items++ )
    ;alloc-or JSR2 ( r|x ) ,&loop JMP
    &done
    ( r ffff )
    POP2
    STHr ,&is-or JCN
    STH2r JMP2r
    &is-or
    POP2r
    ;alloc-empty JSR2 OVR2 OVR2 SWP2 ( r empty empty r )
    ;set-next-or JSR2
    JMP2r

( add r to the top of the stock. )
( )
( in particular, this will write r into tail.next )
( before replacing tail with r. )
@push-next ( r0 r1 -> )
    ;pop4 JSR2 ( r0 r1 x0 x1 )
    DUP2 #0000 EQU2 ( r0 r1 x0 x1 x1=0? ) ,&is-zero JCN
    STH2 ROT2 STH2r ( r1 x0 r0 x1 )
    ;set-next JSR2 SWP2 ( x0 r1 )
    ;push4 JSR2
    JMP2r
    &is-zero POP2 POP2 ;push4 JSR2 JMP2r

( load the given address:  )
( )
(  1. if it points to 0000, update it to target )
(  2. otherwise, call set-next on it )
@set-next-addr ( target* addr* -> )
    LDA2k #0000 EQU2 ( target addr addr=0? ) ,&is-zero JCN
    LDA2 ;set-next JSR2 JMP2r
    &is-zero STA2 JMP2r

( set regex.next to target )
@set-next ( target* regex* -> )
        LDAk #01 NEQ ,&!1 JCN       INC2 ;set-next-addr JSR2 JMP2r
    &!1 LDAk #02 NEQ ,&!2 JCN       INC2 ;set-next-addr JSR2 JMP2r
    &!2 LDAk #03 NEQ ,&!3 JCN #0002 ADD2 ;set-next-addr JSR2 JMP2r
    &!3 LDAk #04 NEQ ,&!4 JCN       INC2 ;set-next-addr JSR2 JMP2r
    &!4 LDAk #05 NEQ ,&!5 JCN #0003 ADD2 ;set-next-addr JSR2 JMP2r
    &!5 ;unknown-node-type ;error! JSR2

@set-next-or-addr ( target* addr* -> )
    LDA2k #0000 EQU2 ( target addr addr=0? ) ,&is-zero JCN
    LDA2 ;set-next-or JSR2 JMP2r
    &is-zero STA2 JMP2r

( this is used when first building or-nodes )
( structure will always be: )
( [x1, [x2, [x3, ..., [xm, xn]]]] )
( so we recurse on the right side but not the left. )
@set-next-or ( target* regex* -> )
    LDAk #04 NEQ ,&!4 JCN
    OVR2 OVR2 INC2 ;set-next-addr JSR2
        #0003 ADD2 ;set-next-or-addr JSR2 JMP2r
    &!4 ;set-next JMP2

( STACK OPERATIONS )
( )
( we always push/pop 4 bytes at a time. the stack has a fixed )
( maximum size it can use, defined by ;stack-top. )
( )
( the stack can be cleared using ;reset-stack, which resets )
( the stack pointers but does not zero out any memory. )
( )
( stack size is 4096 bytes here but is configurable. )
( in some cases it could be very small but this will limit )
( how many branches can be parsed and executed. )

( push 4 bytes onto the stack )
@push4 ( str* regex* -> )
    ;assert-stack-avail JSR2 ( check for space )
    ;stack-pos LDA2 #0002 ADD2 STA2 ( cell[2:3] <- regex )
    ;stack-pos LDA2 STA2 ( cell[0:1] <- str )
    ;stack-pos LDA2 #0004 ADD2 ;stack-pos STA2 ( pos += 4 )
    JMP2r

( pop 4 bytes from the stack )
@pop4 ( -> str* regex* )
    ;assert-stack-exist JSR2 ( check for space )
    ;stack-pos LDA2 ( load stack-pos )
    #0002 SUB2 LDA2k STH2 ( pop and stash regex )
    #0002 SUB2 LDA2k STH2 ( pop and stash str )
    ;stack-pos STA2 ( save new stack-pos )
    STH2r STH2r ( restore str and regex )
    JMP2r

( -> size^ )
@frame-size
    #00 STH ;stack-pos LDA2
    &loop
    #0004 SUB2 LDA2k #ffff EQU2 ,&done JCN
    INCr ,&loop JMP
    &done
    STHr JMP2r

( reset stack pointers )
@reset-stack ( -> )
    ;stack-bot ;stack-pos STA2 JMP2r ( pos <- 0 )

( can more stack be allocated? )
@stack-avail ( -> bool^ )
    ;stack-pos LDA2 ;stack-top LTH2 JMP2r

( is the stack non-empty? )
@stack-exist ( -> bool^ )
    ;stack-pos LDA2 ;stack-bot GTH2 JMP2r

( error if stack is full )
@assert-stack-avail ( -> )
    ;stack-avail JSR2 ,&ok JCN ;stack-is-full ;error! JSR2 &ok JMP2r

( error is stack is empty )
@assert-stack-exist ( -> )
    ;stack-exist JSR2 ,&ok JCN ;stack-is-empty ;error! JSR2 &ok JMP2r

( stack-pos points to the next free stack position (or the top if full). )
@stack-pos :stack-bot ( the next position to insert at )

( stack-bot is the address of the first stack position. )
( stack-top is the address of the first byte beyond the stack. )
@stack-bot $1000 @stack-top ( holds 1024 steps (4096 bytes) )

( ARENA OPERATIONS )
( )
( the arena represents a heap of memory that can easily be )
( allocated in small amounts. )
( )
( the entire arena can be reclaimed using ;reset-arena, but )
( unlike systems such as malloc/free, the arena cannot relcaim )
( smaller amounts of memory. )
( )
( the arena is used to allocate regex graph nodes, which are )
( dynamically-allocated as the regex string is parsed. once )
( a regex is no longer needed the arena may be reclaimed. )
( )
( arena size is 1024 bytes here but is configurable. )
( smaller sizes would likely be fine but will limit the )
( overall complexity of regexes to be parsed and executed. )

( reclaim all the memory used by the arena )
@reset-arena ( -> )
    ;arena-bot ;arena-pos STA2 JMP2r

( currently caller is responsible for zeroing out memory if needed )
@alloc ( size^ -> addr* )
    #00 SWP ( size* )
    ;arena-pos LDA2 STH2k ADD2 ( pos+size* [pos] )
    DUP2 ;arena-top GTH2 ( pos+size pos+size>top? [pos] )
    ,&error JCN ( pos+size [pos] )
    ;arena-pos STA2 ( pos += size [pos] )
    STH2r JMP2r ( pos )
    &error POP2 POP2r ;arena-is-full ;error! JSR2

@arena-pos :arena-bot ( the next position to allocate )
@arena-bot $400 @arena-top ( holds up to 1024 bytes )

## repl-regex.tal
( repl-regex.tal )

%dbg { #ff #0e DEO }
%sp { #20 #18 DEO }
%nl { #0a #18 DEO }
%exit { #01 #0f DEO BRK }

( read in regular expressions )
( and emit internal structures parsed )
|0100
    ;r-prompt ;println JSR2
    ;r-read-stdin #10 DEO2 BRK

( we use two different prompts depending on what mode we're in )
@r-prompt "enter 20 "regex: 20 00
@s-prompt "string 20 "to 20 "match: 20 00


@regex $2     ( compiled regex address (if any) )
@buffer $1000 ( buffer to read user input )
@ptr :buffer  ( next byte to write in buffer )

@println ( s* -> )
    &loop LDAk #00 EQU ,&eof JCN
    LDAk #18 DEO INC2 ,&loop JMP
    &eof POP2 JMP2r

@r-read-stdin ( -> )
    #12 DEI #0a EQU ,&execute JCN
    #12 DEI ;ptr LDA2 STA
    ;ptr LDA2k INC2 SWP2 STA2
    BRK
    &execute
    #00 ;ptr LDA2 STA
    ;buffer ;ptr STA2
    ;buffer ;compile JSR2 dbg nl
    DUP2 ;regex STA2
    ;emit-stack JSR2 nl
    ;emit-arena JSR2 nl
    ;reset-arena JSR2
    POP2
    ;s-prompt ;println JSR2
    ;s-read-stdin #10 DEO2 BRK
    BRK

@s-read-stdin ( -> )
    #12 DEI #0a EQU ,&execute JCN
    #12 DEI ;ptr LDA2 STA
    ;ptr LDA2k INC2 SWP2 STA2
    BRK
    &execute
    #00 ;ptr LDA2 STA ( null terminate string )
    ;ptr LDA2 ;buffer EQU2 STH ( stash is-empty? )
    ;buffer ;ptr STA2 ( reset ptr )
    ;buffer ;regex LDA2 ;match JSR2 ( match regex )
    ;emit-byte JSR2 nl ( print result )
    STHr ,&was-empty JCN
    ;s-prompt ;println JSR2
    BRK
    &was-empty
    ;r-prompt ;println JSR2
    ;r-read-stdin #10 DEO2 BRK
    BRK

~regex.tal

@emit-short ( short* -- )
    SWP ;emit-byte JSR2 ;emit-byte JSR2 JMP2r

@emit-byte ( byte^ -- )
    DUP #04 SFT ,&hex JSR #0f AND ,&hex JMP
    &hex #30 ADD DUP #39 GTH #27 MUL ADD emit
    JMP2r

( print stack size, followed by contents )
@emit-stack ( -> )
    space LIT 'n emit LIT '= emit ;stack-pos LDA2 ;stack-bot SUB2 #0004 DIV2 ;emit-short JSR2 LIT ': emit
    ;stack-bot
    &loop
    DUP2 ;stack-pos LDA2 LTH2 ,&ok JCN
    POP2 newline JMP2r
    &ok
    space LDA2k ;emit-short JSR2
    #0002 ADD2 ,&loop JMP

( emit n bytes from the given address )
@emit-n ( addr* count^ -> addr2* )
    DUP #00 GTH ( addr count count>0? ) ,&ok JCN ( addr count ) POP newline JMP2r
    &ok
    STH ( addr [count] ) space LDAk ;emit-byte JSR2 INC2 ( addr+1 [count] )
    STHr #01 SUB ( addr+1 count-1 )
    ;emit-n JMP2

( emit the arena, with one line per node )
( parses node type, since node size is dynamic (3-5). )
@emit-arena ( -> )
    ;arena-bot
    &loop
    DUP2 ;arena-pos LDA2 LTH2 ,&ok JCN POP2 JMP2r
    &ok
    DUP2 ;emit-short JSR2
    LIT ': emit space
        LDAk #01 NEQ ,&!1 JCN #03 ;emit-n JSR2 ,&loop JMP
    &!1 LDAk #02 NEQ ,&!2 JCN #03 ;emit-n JSR2 ,&loop JMP
    &!2 LDAk #03 NEQ ,&!3 JCN #04 ;emit-n JSR2 ,&loop JMP
    &!3 LDAk #04 NEQ ,&!4 JCN #05 ;emit-n JSR2 ,&loop JMP
    &!4 LDAk #05 NEQ ,&!5 JCN #05 ;emit-n JSR2 ,&loop JMP
    &!5 ;unknown-node-type ;error! JSR2

## test-regex.tal
%dbg { #ff #0e DEO }
%sp { #20 #18 DEO }
%nl { #0a #18 DEO }
%exit { #01 #0f DEO BRK }

|0100
    ;expr1 ;compile JSR2 dbg nl
    ;emit-stack JSR2 nl
    ;emit-arena JSR2 nl

    ;test1 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test2 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test3 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test4 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test5 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test6 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test7 OVR2k ;match JSR2 ;emit-byte JSR2 sp
    ;test8 OVR2k ;match JSR2 ;emit-byte JSR2 nl

    ;test1 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test2 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test3 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test4 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test5 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test6 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test7 ;graph1 ;match JSR2 ;emit-byte JSR2 sp
    ;test8 ;graph1 ;match JSR2 ;emit-byte JSR2 nl

    ;reset-arena JSR2
    exit

( corresponds to regex: a(b|c)d* )
@expr1 "a(b|c)d* 00

( corresponds to regex: a(b|c)d* )
( accepts "ab" or "ac" followd by any number of d's )
@graph1
        03 'a :x1
    @x1 04 :x2 :x3
    @x2 03 'b :x4
    @x3 03 'c :x4
    @x4 05 :x5 0000
    @x5 03 'd :x4

( test case strings to try matching )
@test1 "ab 00 ( yes )
@test2 "acdd 00 ( yes )
@test3 "add 00 ( no )
@test4 "abd 00 ( yes )
@test5 "acddddddddddd 00 ( yes )
@test6 "bd 00 ( no )
@test7 "z 00 ( no )
@test8 00 ( no )

~regex.tal

@emit-short ( short* -- )
    SWP ;emit-byte JSR2 ;emit-byte JSR2 JMP2r

@emit-byte ( byte^ -- )
    DUP #04 SFT ,&hex JSR #0f AND ,&hex JMP
    &hex #30 ADD DUP #39 GTH #27 MUL ADD emit
    JMP2r

( print stack size, followed by contents )
@emit-stack ( -> )
    space LIT 'n emit LIT '= emit ;stack-pos LDA2 ;stack-bot SUB2 #0004 DIV2 ;emit-short JSR2 LIT ': emit
    ;stack-bot
    &loop
    DUP2 ;stack-pos LDA2 LTH2 ,&ok JCN
    POP2 newline JMP2r
    &ok
    space LDA2k ;emit-short JSR2
    #0002 ADD2 ,&loop JMP

( emit n bytes from the given address )
@emit-n ( addr* count^ -> addr2* )
    DUP #00 GTH ( addr count count>0? ) ,&ok JCN ( addr count ) POP newline JMP2r
    &ok
    STH ( addr [count] ) space LDAk ;emit-byte JSR2 INC2 ( addr+1 [count] )
    STHr #01 SUB ( addr+1 count-1 )
    ;emit-n JMP2

( emit the arena, with one line per node )
( parses node type, since node size is dynamic (3-5). )
@emit-arena ( -> )
    ;arena-bot
    &loop
    DUP2 ;arena-pos LDA2 LTH2 ,&ok JCN POP2 JMP2r
    &ok
    DUP2 ;emit-short JSR2
    LIT ': emit space
        LDAk #01 NEQ ,&!1 JCN #03 ;emit-n JSR2 ,&loop JMP
    &!1 LDAk #02 NEQ ,&!2 JCN #03 ;emit-n JSR2 ,&loop JMP
    &!2 LDAk #03 NEQ ,&!3 JCN #04 ;emit-n JSR2 ,&loop JMP
    &!3 LDAk #04 NEQ ,&!4 JCN #05 ;emit-n JSR2 ,&loop JMP
    &!4 LDAk #05 NEQ ,&!5 JCN #05 ;emit-n JSR2 ,&loop JMP
    &!5 ;unknown-node-type ;error! JSR2
	( grep.tal )
	( )
	( by d_m )

	( print a character to STDOUT )
	%emit { #18 DEO }

	( the first argument to grep is the regex )
	( arguments are passed on STDIN, so we just )
	( assume the first "line" is the argument )
	( and the rest are lines to be grepped )
	\|0100
	;r-read-stdin #10 DEO2 BRK

	@regex 0000 ( compiled regex address (if any) )
	@buffer $1000 ( buffer to read user input )
	@ptr :buffer ( next byte to write in buffer )

	@println ( s* -> )
	&loop LDAk #00 EQU ,&eof JCN ( did we reach \0 ? )
	LDAk emit INC2 ,&loop JMP ( no so emit a char and repeat )
	&eof #0a emit POP2 JMP2r ( yes so emit \n and return )

	@r-read-stdin ( -> )
	#12 DEI #0a EQU ,&execute JCN ( did we read \n ? )
	#12 DEI ;ptr LDA2 STA ( no, so save in buffer )
	;ptr LDA2k INC2 SWP2 STA2 ( ptr++ )
	BRK ( return )

	&execute ( we saw a newline, so do something )
	#00 ;ptr LDA2 STA ( null terminate str )
	;buffer ;ptr STA2 ( reset ptr )
	;regex LDA2 #0000 EQU2 ( is regex initialized? )
	,&need-regex JCN ( jump if unset )

	( regex is set )
	;buffer ;regex LDA2 ;match JSR2 ( match regex )
	#00 EQU ,&no-match JCN ( did we match? )
	;buffer ;println JSR2 ( print any match )
	&no-match BRK ( return )

	( regex is unset )
	&need-regex ;buffer ;compile JSR2 ( compile regex )
	;regex STA2 BRK ( save regex and return )

	( include the actual regex machinery )
	~regex.tal
	( regex.tal )
	( )
	( compiles regex expression strings into regex nodes, then uses )
	( regex ndoes to match input strings. )
	( )
	( this currently only supports matching an entire string, as )
	( opposed to searching for a matching substring, or extracting )
	( matching subgroups. )
	( )
	( regex node types: )
	( )
	( NAME DESCRIPTION STRUCT )
	( empty matches empty string [ #01 next* ] )
	( dot matches any one char [ #02 next* ] )
	( lit matches one specific char (c) [ #03 c^ next* ] )
	( or matches either left or right [ #04 left* right* ] )
	( star matches expr zero-or-more times [ #05 r* next* ] )
	( (NOTE: r.expr.next must be r) )
	( )
	( `or` and `star` have the same structure and are handled by the )
	( same code (;do-or). however, the node types are kept different )
	( to make it clearer how to parse and assemble the nodes. )
	( )
	( concatenation isn't a node, it is implied by the *next addr. )
	( a next value of #0000 signals the end of the regex. )
	( )
	( in these docs str* is an address to a null-terminated string. )
	( regexes should not include nulls and cannot match them (other )
	( than the null which signals the end of a string). )

	%null? { #00 EQU }
	%debug { #ff #0e DEO }
	%emit { #18 DEO }
	%space { #20 emit }
	%newline { #0a emit }
	%quit! { #01 #0f DEO BRK }

	( ERROR HANDLING )

	( using error! will print the given message before causing )
	( the interpreter to halt. )
	@error! ( msg* -> )
	LIT '! emit space
	&loop LDAk ,&continue JCN ,&done JMP
	&continue LDAk emit INC2 ,&loop JMP
	&done POP2 newline quit!

	( error messages )
	@unknown-node-type "unknown 20 "node 20 "type 00
	@mismatched-parens "mismatched 20 "parenthesis 00
	@stack-is-full "stack 20 "is 20 "full 00
	@stack-is-empty "stack 20 "is 20 "empty 00
	@arena-is-full "arena 20 "is 20 "full 00
	@star-invariant "star 20 "invariant 20 "failed 00
	@plus-invariant "plus 20 "invariant 20 "failed 00
	@qmark-invariant "question 20 "mark 20 "invariant 20 "failed 00

	( REGEX MATCHING )

	( use stored regex to match against a stored string. )
	( )
	( regex* should be the address of a compiled regex )
	( such as that returned from ;compile. )
	( )
	( str* should be a null-terminated string. )
	( )
	( returns true if the string, and false otherwise. )
	@match ( str* regex* -> bool^ )
	;reset-stack JSR2
	;loop JMP2

	( loop used during matching )
	( )
	( we don't use the return stack here since that )
	( complicates the back-tracking we need to do. )
	( ultimately this code will issue a JMP2r to )
	( return a boolean, which is where the stack )
	( effects signature comes from. )
	@loop ( s* r* -> bool^ )
	LDAk #01 EQU ;do-empty JCN2
	LDAk #02 EQU ;do-dot JCN2
	LDAk #03 EQU ;do-literal JCN2
	LDAk #04 EQU ;do-or JCN2
	LDAk #05 EQU ;do-or JCN2 ( same code as the or case )
	;unknown-node-type ;error! JSR2

	( used when we hit a dead-end during matching. )
	( )
	( if stack is non-empty we have a point we can resume from. )
	@goto-backtrack ( -> bool^ )
	;stack-exist JSR2 ,&has-stack JCN ( do we have stack? )
	#00 JMP2r ( no, return false )
	&has-stack ;pop4 JSR2 ;goto-next JMP2 ( yes, resume from the top )

	( follow the given address (next*) to continue matching )
	@goto-next ( str* next* -> bool^ )
	DUP2 #0000 GTH2 ,&has-next JCN
	POP2 LDA null? ,&end-of-string JCN
	;goto-backtrack JMP2
	&end-of-string #01 JMP2r
	&has-next ;loop JMP2

	( handle the empty node -- just follow the next pointer )
	@do-empty ( str* regex* -> bool^ )
	INC2 LDA2 ( load next )
	;goto-next JMP2 ( jump to next )

	( handle dot -- match any one character )
	@do-dot ( str* regex* -> bool^ )
	INC2 LDA2 STH2 ( load and stash next )
	LDAk #00 NEQ ,&non-empty JCN ( is there a char? )
	POP2r POP2 ;goto-backtrack JMP2 ( no, clear stacks and backtrack )
	&non-empty INC2 STH2r ;goto-next JMP2 ( yes, inc s, restore and jump )

	( handle literal -- match one specific character )
	@do-literal ( str* regex* -> bool^ )
	INC2
	LDAk STH ( store c )
	INC2 LDA2 STH2 ROTr ( store next, move c to top )
	LDAk
	STHr EQU ,&matches JCN ( do we match this char? )
	POP2r POP2 ;goto-backtrack JMP2 ( no, clear stacks and backtrack )
	&matches
	INC2 STH2r ;goto-next JMP2 ( yes, inc s, restore and jump )

	( handle or -- try the left branch but backtrack to the right if needed )
	( )
	( this also handles asteration, since it ends up having the same structure )
	@do-or ( str* regex* -> bool^ )
	INC2 OVR2 OVR2 #0002 ADD2 ( s r+1 s r+3 )
	LDA2 ;push4 JSR2 ( save (s, right) in the stack for possible backtracking )
	LDA2 ;loop JMP2 ( continue on left branch )

	( REGEX PARSING )

	( track the position in the input string )
	@pos $2

	( track how many levels deep we are in parenthesis )
	@parens $2

	( read and increment pos )
	@read ( -> c^ )
	;pos LDA2k ( pos s )
	LDAk STHk #00 EQU ( pos s c=0 [c] )
	,&is-eof JCN ( pos s [c] )
	INC2 ( pos s+1 [c] )
	SWP2 STA2 ,&return JMP ( [c] )
	&is-eof POP2 POP2
	&return STHr ( c )
	JMP2r

	( is pos currently pointing to a star? )
	@peek-to-star ( -> is-star^ )
	;pos LDA2 LDA LIT '* EQU JMP2r

	( is pos currently pointing to a plus? )
	@peek-to-plus ( -> is-plus^ )
	;pos LDA2 LDA LIT '+ EQU JMP2r

	( is pos currently pointing to a qmark? )
	@peek-to-qmark ( -> is-qmark^ )
	;pos LDA2 LDA LIT '? EQU JMP2r

	( just increment pos )
	@skip
	;pos LDA2 INC2 ;pos STA2 JMP2r

	( TODO: )
	( 1. character groups: [] and [^] )
	( 2. symbolic escapes, e.g. \n )

	( STRETCH GOALS: )
	( a. ^ and $ )
	( b. counts: {n} and {m,n} )
	( c. substring matching, i.e. searching )
	( d. subgroup extraction )
	( e. back-references, e.g \1 )

	( compile an expression string into a regex graph )
	( )
	( the regex will be allocated in the arena; if there is not )
	( sufficient space an error will be thrown. )
	( )
	( the stack will also be used during parsing although unlike )
	( the arena it will be released once compilation ends. )
	@compile ( expr* -> regex* )
	;pos STA2
	#0000 ;parens STA2
	;reset-stack JSR2
	;compile-region JMP2

	( the basic strategy here is to build a stack of non-or )
	( expressions to be joined together at the end of the )
	( region. each stack entry has two regex addresses: )
	( - the start of the regex )
	( - the current tail of the regex )
	( when we concatenate a new node to a regex we update )
	( the second of these but not the first. )
	( )
	( the bottom of the stack for a given region is denoted )
	( by #ffff #ffff. above that we start with #0000 #0000 )
	( to signal an empty node. )
	@compile-region ( -> r2* )
	#ffff #ffff ;push4 JSR2 ( stack delimiter )
	#0000 #0000 ;push4 JSR2 ( stack frame start )
	@compile-region-loop
	;read JSR2
	DUP #00 EQU ;c-done JCN2
	DUP LIT '\| EQU ;c-or JCN2
	DUP LIT '. EQU ;c-dot JCN2
	DUP LIT '( EQU ;c-lpar JCN2
	DUP LIT ') EQU ;c-rpar JCN2
	DUP LIT '\ EQU ;c-esc JCN2
	DUP LIT '* EQU ;c-star JCN2
	DUP LIT '+ EQU ;c-plus JCN2
	DUP LIT '? EQU ;c-qmark JCN2
	;c-char JMP2

	( either finalize the given r0/r1 or else wrap it in )
	( a star node if a star is coming up next. )
	( )
	( we use this look-ahead approach rather than compiling )
	( star nodes directly since the implementation is simpler. )
	@c-peek-and-finalize ( r0* r1* -> r2* )
	;peek-to-star JSR2 ( r0 r1 next-is-star? ) ,&next-is-star JCN
	;peek-to-plus JSR2 ( r0 r1 next-is-plus? ) ,&next-is-plus JCN
	;peek-to-qmark JSR2 ( r0 r1 next-is-qmark? ) ,&next-is-qmark JCN
	,&finally JMP ( r0 r1 )
	&next-is-star ;skip JSR2 POP2 ;alloc-star JSR2 DUP2 ,&finally JMP
	&next-is-plus ;skip JSR2 POP2 ;alloc-plus JSR2 DUP2 ,&finally JMP
	&next-is-qmark ;skip JSR2 POP2 ;alloc-qmark JSR2 DUP2 ,&finally JMP
	&finally ;push-next JSR2 ;compile-region-loop JMP2

	( called when we reach EOF of the input string )
	( )
	( as with c-rpar we have to unroll the current level )
	( of the stack, building any or-nodes that are needed. )
	( )
	( this is where we detect unclosed parenthesis. )
	@c-done ( c^ -> r2* )
	POP
	;parens LDA2 #0000 GTH2 ,&mismatched-parens JCN
	;unroll-stack JSR2 POP2 JMP2r
	&mismatched-parens ;mismatched-parens ;error! JSR2

	( called when we read "\|" )
	( )
	( since we defer building or-nodes until the end of the region )
	( we just start a new stack frame and continue. )
	@c-or ( c^ -> r2* )
	POP
	#0000 #0000 ;push4 JSR2
	;compile-region-loop JMP2

	( called when we read "(" )
	( )
	( this causes us to: )
	( )
	( 1. increment parens )
	( 2. start a new region on the stack )
	( 3. jump to compile-region to start parsing the new region )
	@c-lpar ( c^ -> r2* )
	POP
	;parens LDA2 INC2 ;parens STA2 ( parens++ )
	;compile-region JMP2

	( called when we read ")" )
	( )
	( this causes us to: )
	( )
	( 1. check for mismatched parens )
	( 2. decrement parens )
	( 3. unroll the current region on the stack into one regex node )
	( 4. finalize that node and append it to the previous region )
	( 5. continue parsing )
	@c-rpar ( c^ -> r2* )
	POP
	;parens LDA2 #0000 EQU2 ,&mismatched-parens JCN
	;parens LDA2 #0001 SUB2 ;parens STA2 ( parens-- )
	;unroll-stack JSR2
	;c-peek-and-finalize JMP2
	&mismatched-parens ;mismatched-parens ;error! JSR2

	( called when we read "." )
	( )
	( allocates a dot-node and continues. )
	@c-dot ( c^ -> r2* )
	POP
	;alloc-dot JSR2 ( dot )
	DUP2 ;c-peek-and-finalize JMP2

	( TODO: escaping rules not quite right )

	( called when we read "\" )
	( )
	( allocates a literal of the next character. )
	( )
	( this doesn't currently handle any special escape sequences. )
	@c-esc ( c^ -> r2* )
	POP
	;read JSR2
	;c-char JMP2

	( called when we read any other character )
	( )
	( allocates a literal-node and continues. )
	@c-char ( c^ -> r2* )
	;alloc-lit JSR2 ( lit )
	DUP2 ;c-peek-and-finalize JMP2

	( called if we parse a "*" )
	( )
	( actually calling this means the code broke an invariant somewhere. )
	@c-star ( c^ -> regex* )
	POP
	;star-invariant ;error! JSR2

	( called if we parse a "+" )
	( )
	( actually calling this means the code broke an invariant somewhere. )
	@c-plus ( c^ -> regex* )
	POP
	;plus-invariant ;error! JSR2

	( called if we parse a "?" )
	( )
	( actually calling this means the code broke an invariant somewhere. )
	@c-qmark ( c^ -> regex* )
	POP
	;qmark-invariant ;error! JSR2

	( ALLOCATING REGEX NDOES )

	@alloc3 ( mode^ -> r* )
	#0000 ROT ( 00 00 mode^ )
	#03 ;alloc JSR2 ( 00 00 mode^ addr* )
	STH2k STA ( addr <- mode )
	STH2kr INC2 STA2 ( addr+1 <- 0000 )
	STH2r JMP2r ( return addr )

	@alloc-empty ( -> r* )
	#01 ;alloc3 JMP2

	@alloc-dot ( -> r* )
	#02 ;alloc3 JMP2

	@alloc-lit ( c^ -> r* )
	#03 #0000 SWP2 ( 0000 c^ 03 )
	#04 ;alloc JSR2 ( 0000 c^ 03 addr* )
	STH2k STA ( addr <- 03 )
	STH2kr INC2 STA ( addr+1 <- c )
	STH2kr #0002 ADD2 STA2 ( addr+2 <- 0000 )
	STH2r JMP2r ( return addr )

	@alloc-or ( right* left* -> r* )
	#05 ;alloc JSR2 STH2 ( r l [x] )
	#04 STH2kr STA ( r l [x] )
	STH2kr INC2 STA2 ( r [x] )
	STH2kr #0003 ADD2 STA2 ( [x] )
	STH2r JMP2r

	@alloc-star ( expr* -> r* )
	#05 ;alloc JSR2 STH2 ( expr [r] )
	#05 STH2kr STA ( expr [r] )
	DUP2 STH2kr INC2 STA2 ( expr [r] )
	#0000 STH2kr #0003 ADD2 STA2 ( expr [r] )
	STH2kr SWP2 ( r expr [r] )
	;set-next JSR2 ( [r] )
	STH2r JMP2r

	@alloc-plus ( expr* -> r* )
	#05 ;alloc JSR2 STH2 ( expr [r] )
	#05 STH2kr STA ( expr [r] )
	DUP2 STH2kr INC2 STA2 ( expr [r] )
	#0000 STH2kr #0003 ADD2 STA2 ( expr [r] )
	STH2r SWP2 STH2k ( r expr [expr] )
	;set-next JSR2 ( [expr] )
	STH2r JMP2r

	@alloc-qmark ( expr* -> r* )
	;alloc-empty JSR2 STH2k ( expr e [e] )
	OVR2 ;set-next JSR2 ( expr [e] )
	#05 ;alloc JSR2 STH2 ( expr [r e] )
	#04 STH2kr STA ( expr [r e] )
	STH2kr INC2 STA2 ( [r e] )
	SWP2r STH2r STH2kr ( e r [r] )
	#0003 ADD2 STA2 ( [r] )
	STH2r JMP2r

	( if r is 0000, allocate an empty node )
	@alloc-if-null ( r* -> r2* )
	ORAk ,&return JCN POP2 ;alloc-empty JSR2 &return JMP2r

	( unroll one region of the parsing stack, returning )
	( a single node consisting of an alternation of )
	( all elements on the stack. )
	( )
	( this unrolls until it hits #ffff #ffff, which it )
	( also removes from the stack. )
	@unroll-stack ( -> start* end* )
	;pop4 JSR2 STH2 ( r )
	#00 STH ( count items in stack frame )
	;alloc-if-null JSR2 ( replace 0000 with empty )
	&loop ( r* )
	;pop4 JSR2 POP2 ( r x )
	DUP2 #ffff EQU2 ( r x x-is-end? ) ,&done JCN
	INCr ( items++ )
	;alloc-or JSR2 ( r\|x ) ,&loop JMP
	&done
	( r ffff )
	POP2
	STHr ,&is-or JCN
	STH2r JMP2r
	&is-or
	POP2r
	;alloc-empty JSR2 OVR2 OVR2 SWP2 ( r empty empty r )
	;set-next-or JSR2
	JMP2r

	( add r to the top of the stock. )
	( )
	( in particular, this will write r into tail.next )
	( before replacing tail with r. )
	@push-next ( r0 r1 -> )
	;pop4 JSR2 ( r0 r1 x0 x1 )
	DUP2 #0000 EQU2 ( r0 r1 x0 x1 x1=0? ) ,&is-zero JCN
	STH2 ROT2 STH2r ( r1 x0 r0 x1 )
	;set-next JSR2 SWP2 ( x0 r1 )
	;push4 JSR2
	JMP2r
	&is-zero POP2 POP2 ;push4 JSR2 JMP2r

	( load the given address: )
	( )
	( 1. if it points to 0000, update it to target )
	( 2. otherwise, call set-next on it )
	@set-next-addr ( target* addr* -> )
	LDA2k #0000 EQU2 ( target addr addr=0? ) ,&is-zero JCN
	LDA2 ;set-next JSR2 JMP2r
	&is-zero STA2 JMP2r

	( set regex.next to target )
	@set-next ( target* regex* -> )
	LDAk #01 NEQ ,&!1 JCN INC2 ;set-next-addr JSR2 JMP2r
	&!1 LDAk #02 NEQ ,&!2 JCN INC2 ;set-next-addr JSR2 JMP2r
	&!2 LDAk #03 NEQ ,&!3 JCN #0002 ADD2 ;set-next-addr JSR2 JMP2r
	&!3 LDAk #04 NEQ ,&!4 JCN INC2 ;set-next-addr JSR2 JMP2r
	&!4 LDAk #05 NEQ ,&!5 JCN #0003 ADD2 ;set-next-addr JSR2 JMP2r
	&!5 ;unknown-node-type ;error! JSR2

	@set-next-or-addr ( target* addr* -> )
	LDA2k #0000 EQU2 ( target addr addr=0? ) ,&is-zero JCN
	LDA2 ;set-next-or JSR2 JMP2r
	&is-zero STA2 JMP2r

	( this is used when first building or-nodes )
	( structure will always be: )
	( [x1, [x2, [x3, ..., [xm, xn]]]] )
	( so we recurse on the right side but not the left. )
	@set-next-or ( target* regex* -> )
	LDAk #04 NEQ ,&!4 JCN
	OVR2 OVR2 INC2 ;set-next-addr JSR2
	#0003 ADD2 ;set-next-or-addr JSR2 JMP2r
	&!4 ;set-next JMP2

	( STACK OPERATIONS )
	( )
	( we always push/pop 4 bytes at a time. the stack has a fixed )
	( maximum size it can use, defined by ;stack-top. )
	( )
	( the stack can be cleared using ;reset-stack, which resets )
	( the stack pointers but does not zero out any memory. )
	( )
	( stack size is 4096 bytes here but is configurable. )
	( in some cases it could be very small but this will limit )
	( how many branches can be parsed and executed. )

	( push 4 bytes onto the stack )
	@push4 ( str* regex* -> )
	;assert-stack-avail JSR2 ( check for space )
	;stack-pos LDA2 #0002 ADD2 STA2 ( cell[2:3] <- regex )
	;stack-pos LDA2 STA2 ( cell[0:1] <- str )
	;stack-pos LDA2 #0004 ADD2 ;stack-pos STA2 ( pos += 4 )
	JMP2r

	( pop 4 bytes from the stack )
	@pop4 ( -> str* regex* )
	;assert-stack-exist JSR2 ( check for space )
	;stack-pos LDA2 ( load stack-pos )
	#0002 SUB2 LDA2k STH2 ( pop and stash regex )
	#0002 SUB2 LDA2k STH2 ( pop and stash str )
	;stack-pos STA2 ( save new stack-pos )
	STH2r STH2r ( restore str and regex )
	JMP2r

	( -> size^ )
	@frame-size
	#00 STH ;stack-pos LDA2
	&loop
	#0004 SUB2 LDA2k #ffff EQU2 ,&done JCN
	INCr ,&loop JMP
	&done
	STHr JMP2r

	( reset stack pointers )
	@reset-stack ( -> )
	;stack-bot ;stack-pos STA2 JMP2r ( pos <- 0 )

	( can more stack be allocated? )
	@stack-avail ( -> bool^ )
	;stack-pos LDA2 ;stack-top LTH2 JMP2r

	( is the stack non-empty? )
	@stack-exist ( -> bool^ )
	;stack-pos LDA2 ;stack-bot GTH2 JMP2r

	( error if stack is full )
	@assert-stack-avail ( -> )
	;stack-avail JSR2 ,&ok JCN ;stack-is-full ;error! JSR2 &ok JMP2r

	( error is stack is empty )
	@assert-stack-exist ( -> )
	;stack-exist JSR2 ,&ok JCN ;stack-is-empty ;error! JSR2 &ok JMP2r

	( stack-pos points to the next free stack position (or the top if full). )
	@stack-pos :stack-bot ( the next position to insert at )

	( stack-bot is the address of the first stack position. )
	( stack-top is the address of the first byte beyond the stack. )
	@stack-bot $1000 @stack-top ( holds 1024 steps (4096 bytes) )

	( ARENA OPERATIONS )
	( )
	( the arena represents a heap of memory that can easily be )
	( allocated in small amounts. )
	( )
	( the entire arena can be reclaimed using ;reset-arena, but )
	( unlike systems such as malloc/free, the arena cannot relcaim )
	( smaller amounts of memory. )
	( )
	( the arena is used to allocate regex graph nodes, which are )
	( dynamically-allocated as the regex string is parsed. once )
	( a regex is no longer needed the arena may be reclaimed. )
	( )
	( arena size is 1024 bytes here but is configurable. )
	( smaller sizes would likely be fine but will limit the )
	( overall complexity of regexes to be parsed and executed. )

	( reclaim all the memory used by the arena )
	@reset-arena ( -> )
	;arena-bot ;arena-pos STA2 JMP2r

	( currently caller is responsible for zeroing out memory if needed )
	@alloc ( size^ -> addr* )
	#00 SWP ( size* )
	;arena-pos LDA2 STH2k ADD2 ( pos+size* [pos] )
	DUP2 ;arena-top GTH2 ( pos+size pos+size>top? [pos] )
	,&error JCN ( pos+size [pos] )
	;arena-pos STA2 ( pos += size [pos] )
	STH2r JMP2r ( pos )
	&error POP2 POP2r ;arena-is-full ;error! JSR2

	@arena-pos :arena-bot ( the next position to allocate )
	@arena-bot $400 @arena-top ( holds up to 1024 bytes )
	( repl-regex.tal )

	%dbg { #ff #0e DEO }
	%sp { #20 #18 DEO }
	%nl { #0a #18 DEO }
	%exit { #01 #0f DEO BRK }

	( read in regular expressions )
	( and emit internal structures parsed )
	\|0100
	;r-prompt ;println JSR2
	;r-read-stdin #10 DEO2 BRK

	( we use two different prompts depending on what mode we're in )
	@r-prompt "enter 20 "regex: 20 00
	@s-prompt "string 20 "to 20 "match: 20 00


	@regex $2 ( compiled regex address (if any) )
	@buffer $1000 ( buffer to read user input )
	@ptr :buffer ( next byte to write in buffer )

	@println ( s* -> )
	&loop LDAk #00 EQU ,&eof JCN
	LDAk #18 DEO INC2 ,&loop JMP
	&eof POP2 JMP2r

	@r-read-stdin ( -> )
	#12 DEI #0a EQU ,&execute JCN
	#12 DEI ;ptr LDA2 STA
	;ptr LDA2k INC2 SWP2 STA2
	BRK
	&execute
	#00 ;ptr LDA2 STA
	;buffer ;ptr STA2
	;buffer ;compile JSR2 dbg nl
	DUP2 ;regex STA2
	;emit-stack JSR2 nl
	;emit-arena JSR2 nl
	;reset-arena JSR2
	POP2
	;s-prompt ;println JSR2
	;s-read-stdin #10 DEO2 BRK
	BRK

	@s-read-stdin ( -> )
	#12 DEI #0a EQU ,&execute JCN
	#12 DEI ;ptr LDA2 STA
	;ptr LDA2k INC2 SWP2 STA2
	BRK
	&execute
	#00 ;ptr LDA2 STA ( null terminate string )
	;ptr LDA2 ;buffer EQU2 STH ( stash is-empty? )
	;buffer ;ptr STA2 ( reset ptr )
	;buffer ;regex LDA2 ;match JSR2 ( match regex )
	;emit-byte JSR2 nl ( print result )
	STHr ,&was-empty JCN
	;s-prompt ;println JSR2
	BRK
	&was-empty
	;r-prompt ;println JSR2
	;r-read-stdin #10 DEO2 BRK
	BRK

	~regex.tal

	@emit-short ( short* -- )
	SWP ;emit-byte JSR2 ;emit-byte JSR2 JMP2r

	@emit-byte ( byte^ -- )
	DUP #04 SFT ,&hex JSR #0f AND ,&hex JMP
	&hex #30 ADD DUP #39 GTH #27 MUL ADD emit
	JMP2r

	( print stack size, followed by contents )
	@emit-stack ( -> )
	space LIT 'n emit LIT '= emit ;stack-pos LDA2 ;stack-bot SUB2 #0004 DIV2 ;emit-short JSR2 LIT ': emit
	;stack-bot
	&loop
	DUP2 ;stack-pos LDA2 LTH2 ,&ok JCN
	POP2 newline JMP2r
	&ok
	space LDA2k ;emit-short JSR2
	#0002 ADD2 ,&loop JMP

	( emit n bytes from the given address )
	@emit-n ( addr* count^ -> addr2* )
	DUP #00 GTH ( addr count count>0? ) ,&ok JCN ( addr count ) POP newline JMP2r
	&ok
	STH ( addr [count] ) space LDAk ;emit-byte JSR2 INC2 ( addr+1 [count] )
	STHr #01 SUB ( addr+1 count-1 )
	;emit-n JMP2

	( emit the arena, with one line per node )
	( parses node type, since node size is dynamic (3-5). )
	@emit-arena ( -> )
	;arena-bot
	&loop
	DUP2 ;arena-pos LDA2 LTH2 ,&ok JCN POP2 JMP2r
	&ok
	DUP2 ;emit-short JSR2
	LIT ': emit space
	LDAk #01 NEQ ,&!1 JCN #03 ;emit-n JSR2 ,&loop JMP
	&!1 LDAk #02 NEQ ,&!2 JCN #03 ;emit-n JSR2 ,&loop JMP
	&!2 LDAk #03 NEQ ,&!3 JCN #04 ;emit-n JSR2 ,&loop JMP
	&!3 LDAk #04 NEQ ,&!4 JCN #05 ;emit-n JSR2 ,&loop JMP
	&!4 LDAk #05 NEQ ,&!5 JCN #05 ;emit-n JSR2 ,&loop JMP
	&!5 ;unknown-node-type ;error! JSR2