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@Zacharý No. They're PCRE, but ⎕R and ⎕S do have some special and nifty syntax.
@J.Sallé You're not allowed to modify ⍺, only (attempt) to assign to it, and that whole statement is skipped if ⍺ has a value. In a way, ⍺←xxx is like a ×⎕NC'a': guard, but execution continues with the next statement regardless.
@J.Sallé Just use a different variable name.
@J.Sallé Yes. Count line breaks as a single byte.
@J.Sallé Yes you can: Guards work by statement, not line, so {Condition:ValueIfTrue ⋄ ValIfFalse} works fine.
