@INSTR[start,[length],string] : Returns a substring, beginning at offset start and continuing for length characters. If length is positive or it is omitted, the offset is measured from the beginning (i.e., left end) of the string. If length is omitted, all of the string beginning at offset start is returned. If length is negative, the offset is measured leftward from the right end of the string, and its length is specified by the value of length without the minus sign. @SUBSTR is an older version of the same function.

Numeric input may be entered in either decimal format (a sequence of 0-9 digits) or in hexadecimal format ("0x" followed by a sequence of 0-F hex digits). To use hexadecimal form for a negative length, remember to use 32-bit 2's complement arithmetic.

Examples:

function |
value |

=%@instr[8,3,this is useful]= |
=use= |

=%@instr[8,,this is useful]= |
=useful= |

=%@instr[8,-4,this is useful]= |
=is u= |

=%@instr[8,,commas, they don't matter]= |
=they don't matter= |