EXPR(1) | NetBSD General Commands Manual | EXPR(1) |
expr | expression |
All operators are separate arguments to the expr utility. Characters special to the command interpreter must be escaped.
Operators are listed below in order of increasing precedence. Operators with equal precedence are grouped within { } symbols.
If the match succeeds and the pattern contains at least one regular expression subexpression “\(...\)”, the string corresponding to “\1” is returned; otherwise the matching operator returns the number of characters matched. If the match fails and the pattern contains a regular expression subexpression the null string is returned; otherwise 0.
Additionally, the following keywords are recognized:
Operator precedence (from highest to lowest):
a=`expr $a + 1`
expr 1 '&' 1 - 1
expr /$a : '.*/\(.*\)'
expr $a : '.*'
expr on other systems (including NetBSD up to and including NetBSD 1.5) might not be so graceful. Arithmetic results might be arbitrarily limited on such systems, most commonly to 32 bit quantities. This means such expr can only process values between -2147483648 and +2147483647.
On other systems, expr might also not work correctly for regular expressions where either side contains single forward slash, like this:
expr / : '.*/\(.*\)'
If this is the case, you might use // (double forward slash) to avoid confusion with the division operator:
expr "//$a" : '.*/\(.*\)'
According to IEEE Std 1003.2 (“POSIX.2”), expr has to recognize special option '--', treat it as an end of command line options and ignore it. Some expr implementations don't recognize it at all, others might ignore it even in cases where doing so results in syntax error. There should be same result for both following examples, but it might not always be:
expr '' : '$'
The reason is that the returned number of matched characters (zero) is indistinguishable from a failed match, so this returns failure. To match the empty string, use something like:
expr x'' : 'x$'
April 20, 2004 | NetBSD 5.99 |