expr is as specified in the Single UNIX Specification but supports extra operands as listed below.
The behaviors specified in this section are expected to disappear from a future version of the LSB; applications should only use the non-LSB-deprecated behaviors.
The following are extra operands:
is same as STRING : REGEXP.
is substring of STRING, POS counted from 1.
is index in STRING where any CHARS is found, or 0.
is length of STRING.
interprets TOKEN as a string, even if it is a keyword like match or an operator like /.