(coreutils.info)String expressions

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16.4.1 String expressions
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‘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.)  Note: Regular
     Expression Library, 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.