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16.4.1 String expressions

expr supports pattern matching and other string operators. These have higher precedence than both the numeric and relational operators (in the next sections).

string : regex
Perform pattern matching. The arguments are converted to strings and the second is considered to be a (basic, a la GNU grep) regular expression, with a ^ implicitly prepended. The first argument is then matched against this regular expression.

If the match succeeds and regex uses ‘\(’ and ‘\)’, the : expression returns the part of string that matched the subexpression; otherwise, it returns the number of characters matched.

If the match fails, the : operator returns the null string if ‘\(’ and ‘\)’ are used in regex, otherwise 0.

Only the first ‘\( ... \)’ pair is relevant to the return value; additional pairs are meaningful only for grouping the regular expression operators.

In the regular expression, \+, \?, and \| are operators which respectively match one or more, zero or one, or separate alternatives. SunOS and other expr's treat these as regular characters. (POSIX allows either behavior.) See Regular Expression Library, for details of regular expression syntax. Some examples are in Examples of expr.

match string regex
An alternative way to do pattern matching. This is the same as ‘string : regex.
substr string position length
Returns the substring of string beginning at position with length at most length. If either position or length is negative, zero, or non-numeric, returns the null string.
index string charset
Returns the first position in string where the first character in charset was found. If no character in charset is found in string, return 0.
length string
Returns the length of string.
+ token
Interpret token as a string, even if it is a keyword like match or an operator like /. This makes it possible to test expr length + "$x" or expr + "$x" : '.*/\(.\)' and have it do the right thing even if the value of $x happens to be (for example) / or index. This operator is a GNU extension. Portable shell scripts should use " $token" : ' \(.*\)' instead of + "$token".

To make expr interpret keywords as strings, you must use the quote operator.