String expressions (GNU Coreutils 9.0)
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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
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 ofstring
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. See Regular Expressions in The GNU Grep Manual, 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
’ /@w .- ‘
substr string position length
’ Returns the substring of
string
beginning atposition
with length at mostlength
. If eitherposition
orlength
is negative, zero, or non-numeric, returns the null string.- ‘
index string charset
’ Returns the first position in
string
where the first character incharset
was found. If no character incharset
is found instring
, return 0.- ‘
length string
’ Returns the length of
string
.- ‘
+ token
’ Interpret
token
as a string, even if it is a keyword likematch
or an operator like/
. This makes it possible to testexpr length + "$x"
orexpr + "$x" : '.*/\(.\)'
and have it do the right thing even if the value of$x
happens to be (for example)/
orindex
. 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.
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