From Xojo Documentation
The byte offset at which you want to start the search if the optional TargetString parameter to Replace is not specified.
Keep in mind: If you set it, it will only be used if you don't specify a TargetString, since setting a new TargetString resets the value.
The offset is zero-based!. I.e. to start at the beginning of the string, use 0.
If you want to set a start value past the first character, and if the string uses an encoding where not every character is exactly one byte long, such as UTF-8, you need to convert the string's character position into its byte offset.
Here’s a way to convert a (1-based) character position into the (0-based) byte position:
aRegEx.SearchStartPosition = theString.Left(characterPosition-1).Bytes