File: coreutils.info,  Node: String expressions,  Next: Numeric expressions,  Up: expr invocation

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 REGEX does not use ‘\(’ and ‘\)’, the ‘:’ expression returns the
     number of characters matched, or 0 if the match fails.

     If REGEX uses ‘\(’ and ‘\)’, the ‘:’ expression returns the part of
     STRING that matched the subexpression, or the null string if the
     match failed or the subexpression did not contribute to the match.

     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.  These operators are GNU extensions.  *Note Regular
     Expressions: (grep)Regular Expressions, for details of regular
     expression syntax.  Some examples are in *note 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.