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3.6 Case Sensitivity in Matching
Case is normally significant in regular expressions, both when matching ordinary characters (i.e., not metacharacters) and inside bracket expressions. Thus, a ‘w’ in a regular expression matches only a lowercase ‘w’ and not an uppercase ‘W’.
The simplest way to do a case-independent match is to use a bracket expression—for example, ‘[Ww]’. However, this can be cumbersome if you need to use it often, and it can make the regular expressions harder to read. There are two alternatives that you might prefer.
One way to perform a case-insensitive match at a particular point in the
program is to convert the data to a single case, using the
tolower()
or toupper()
built-in string functions (which we
haven’t discussed yet;
see section String-Manipulation Functions).
For example:
tolower($1) ~ /foo/ { … } |
converts the first field to lowercase before matching against it.
This works in any POSIX-compliant awk
.
Another method, specific to gawk
, is to set the variable
IGNORECASE
to a nonzero value (see section Built-in Variables).
When IGNORECASE
is not zero, all regexp and string
operations ignore case. Changing the value of
IGNORECASE
dynamically controls the case-sensitivity of the
program as it runs. Case is significant by default because
IGNORECASE
(like most variables) is initialized to zero:
x = "aB" if (x ~ /ab/) … # this test will fail IGNORECASE = 1 if (x ~ /ab/) … # now it will succeed |
In general, you cannot use IGNORECASE
to make certain rules
case-insensitive and other rules case-sensitive, because there is no
straightforward way
to set IGNORECASE
just for the pattern of
a particular rule.(17)
To do this, use either bracket expressions or tolower()
. However, one
thing you can do with IGNORECASE
only is dynamically turn
case-sensitivity on or off for all the rules at once.
IGNORECASE
can be set on the command line or in a BEGIN
rule
(see section Other Command-Line Arguments; also
see section Startup and Cleanup Actions).
Setting IGNORECASE
from the command line is a way to make
a program case-insensitive without having to edit it.
Both regexp and string comparison
operations are affected by IGNORECASE
.
In multibyte locales, the equivalences between upper- and lowercase characters are tested based on the wide-character values of the locale’s character set. Otherwise, the characters are tested based on the ISO-8859-1 (ISO Latin-1) character set. This character set is a superset of the traditional 128 ASCII characters, which also provides a number of characters suitable for use with European languages.(18)
The value of IGNORECASE
has no effect if gawk
is in
compatibility mode (see section Command-Line Options).
Case is always significant in compatibility mode.
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