INDEX ( STRING, SUBSTRING [ , BACK , KIND ] )

STRING : Character, any kind;
SUBSTRING : same type and kind as STRING;
BACK : Logical;
KIND : scalar Integer constant expression;

Searches STRING for an occurrence of SUBSTRING. If BACK is absent, or present with the value .FALSE., the search begins at the start of STRING; if BACK is present with the value .TRUE., the search begins at the end of STRING.

If there are no occurrences of SUBSTRING in STRING, the value zero is returned; otherwise, the result I has the property that STRING(I:I+LEN(SUBSTRING)-1)==SUBSTRING, and is the least such value for a forwards search and the greatest such value for a backwards search.

This means that if SUBSTRING is length zero, the result is equal to 1 for a forwards search and LEN(STRING)+1 for a backwards search.

Like many intrinsic functions, this is elemental; if any of STRING, SUBSTRING or BACK is an array, the result is also an array with the same shape and the operation being applied elementwise. Note that all array arguments must have the same shape.