A second numeric argument targets a particular position in the string. An optional third number specifies the number of characters to select. If the first string argument in the node test is the empty string, then relevant positions in the context node's text contents will be selected. When matching strings, case is considered.
White space is condensed to a single space.
Markup characters are ignored. The most common use of the string axis is to select part of the text contents of a node previously selected by more conventional means, rather than to pick an absolute location in the document.