The following syntax is used for the text input of the Guarded Commands Code, starting with a command C.

C ::=

x := a | A[a] := a | read x | read A[a] | write a | skip | C; C | if GC fi | do GC od | continue | break | try C catch HC yrt | throw e

GC ::=

b -> C | GC [] GC

HC ::=

e: C | HC [] HC

a ::=

n | x | A[a] | a + a | a - a | a * a | a / a | a % a | -a | (a)

b ::=

true | false | b & b | b | b | b && b | b || b | !b | a = a | a != a | a > a | a >= a | a < a | a <= a | (b)

Additional notes:

• && and || do short-circut evaluation (i.e. a minimal number of operands are evaluated left to right), the rest of the operators do eager evaluation (i.e. all operands are evaluted).
• Precedence and associativity rules:
  • Precedence is highest for - (unary minus), then *, % and /, and lowest for + and -.
  • Precedence is highest for !, then & and &&, and lowest for | and ||.
  • *, %, /, +, -, &, |, &&, and || associate left to right.
  • !, - (unary minus), [], and ; associate right to left.
• Variables x, arrays A and exceptions e match the regular expression [a-zA-Z][a-zA-Z\d_]* and can not be any of the keywords. Furthermore the three sets should be pair-wise disjoint, e.g. a variable identifier can not be used both as a variable and an array (and an exception) in the same program.
• Numbers n match the regular expression \d+ and are bound by 2⁵³-1 (JavaScript Number.MAX_SAFE_INTEGER). Negative numbers are passed as unary minus -n.
• A whitespace matches the regular expression [ \u00A0\f\n\r\t\v], with a mandatory whitespace after read␣, write␣, if␣, do␣, try␣, throw␣, before ␣fi, ␣od, ␣yrt, and around ␣catch␣. Whitespaces are ignored anywhere else.