start

Optional. Numeric expression that sets the starting position for each search. The first character position is position 1.

If this argument is omitted, search begins at the first character position. If start contains Null, an error occurs. The start argument is required if compare is specified.

string1

Required. String expression being searched.

string2

Required. String expression searched for.

compare

Optional. Numeric value indicating the kind of comparison to use when evaluating substrings. See Settings section for values. If omitted, a binary comparison is performed.

The compare argument can have the following values:

The InStr function returns the following values:

The following examples use InStr to search a string:

Dim MyPos

' This is a binary comparison because the last argument is omitted.
' Returns 3.
MyPos = InStr ("abcdefgh", "c")

' In this binary comparison, the uppercase "C" is not found
' in the lowercase string.
' Returns 0 (not found).
MyPos = InStr ("abcdefgh", "C")

' In this text comparison starting at the first position,
' the uppercase "C" is found in the lowercase string.
' Returns 3.
MyPos = InStr (1, "abcdefgh", "C", vbTextCompare)

Note:
The InStrB function is used with byte data contained in a string. Instead of returning the character position of the first occurrence of one string within another, InStrB returns the byte position.

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Reference