Tcl_ParseCommand - function in Tcl's public C API for parsing a
Tcl command from a string.

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Official docs

http://www.purl.org/tcl/home/man/tcl8.4/TclLib/ParseCmd.htm
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  int
  Tcl_ParseCommand(
      Tcl_Interp *interp,
      const char *string,
      int         numBytes,
      int         nested,
      Tcl_Parse  *parsePtr)

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There have been several bugs deep in the Tcl core dealing
with the parsing of commands that suggest the existing
docs for '''Tcl_ParseCommand''' don't point out a tricky
case well enough.  See Tcl Bug 681641.

The end of a Tcl command is determined by the presence
of a command terminator character.  This might be a
newline ("\n") or a semi-colon (";").  When Tcl_ParseCommand
is asked to parse a command in a command substitution
context (by setting the ''nested'' value to true), then
the close-bracket character ("]") is also a command terminator.

Tcl_ParseCommand returns its parsing results in a Tcl_Parse
structure pointed to by ''parsePtr''.