XML in a Nutshell, 3rd Edition

Strings

XPath strings are ordered sequences of Unicode characters such as “Fred”, “Ethel”, " ل ا ه ر“, or “”. String literals may be enclosed in either single or double quotes as convenient. The quotes are not themselves part of the string. The only restriction XPath places on a string literal is that it must not contain the kind of quote that delimits it. That is, if the string contains single quotes, it has to be enclosed in double quotes and vice versa. String literals may contain whitespace including tabs, carriage returns, and line feeds, as well as backslashes and other characters that would be illegal in many programming languages. However, if the XPath expression is part of an XML document, some characters may need to be escaped to satisfy XML’s well-formedness rules.

You can use the = and != comparison operators to check whether two strings are the same. You can also use the relational <, >, <=, and >= operators to compare strings, but unless both strings clearly represent numbers (e.g., "-7.5" or '54.2'), the results are unlikely to make sense. In general, you can’t define any real notion of string order in Unicode without detailed knowledge of the language in which the string is written.

Other operations on strings are provided by XPath functions and will be discussed shortly.