Interesting problem. A simple regexp-based solution that doesn't take lexemes into account will fail, at least on pathological cases like 1800 IF V$="THEN FOO ELSE IF BAR" THEN $K="FOO" I don't have the brain cells left tonight to work through a complete solution, though I have a dim recollection of having done something like this many years back using "fixups". 1800 IF V$="K" THEN A$="+K+" ELSE IF V$="R" THEN A$="?R?" ELSE IF V$="M" THEN A$="!M!":Z1=R1:Z2=R2 would translate first into

1800   IF V$="K" THEN GOTO {fixup:skip}
1800.1 GOTO {fixup:after-next-goto}
1800.2 A$="+K+"
1800.3 GOTO {fixup:end}
1800.4 IF V$="R" THEN GOTO {fixup:skip}
1800.5 GOTO {fixup:after-next-goto}
1800.6 A$="?R?"
1800.7 GOTO {fixup:end}
1800.8 IF V$="M" THEN GOTO {fixup:skip}
1800.9 GOTO {fixup:end}
1800.a A$="!M!"
1800.b Z1=R1
1800.c Z2=R2
1800.d REM
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The second pass would peform the fixups.

• {fixup:skip} becomes the number of the 2nd following line in the sequence
• {fixup:end} becomes the final line number of the sequence
• {fixup:after-next-goto} becomes the line number after the next goto in the sequence. (This works because conditionals cannot be nested within a line.)

By using a {fixup:skip} fixup (rather than calculating the target line number as you're generating the sequence, you allow for the possibility of doing peephole optimizations. In the sequence above,

1800.8 IF V$="M" THEN GOTO {fixup:skip}
1800.9 GOTO {fixup:end}
1800.a
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could be optimized to

1800.8 IF V$<>"M" THEN GOTO {fixup:end}
1800.9
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This "renumbers" lines within the sequence, but since target line number calculation/assignment has been deferred, not GOTOs are broken.