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GNU Info (gawk.info)Case-sensitivityCase Sensitivity in Matching ============================ Case is normally significant in regular expressions, both when matching ordinary characters (i.e., not metacharacters) and inside character sets. 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 character list--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; Note: 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 (Note: 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.(1) To do this, use either character lists 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 (Note: Other Command-Line Arguments.; also Note: Startup and Cleanup Actions.). Setting `IGNORECASE' from the command line is a way to make a program case-insensitive without having to edit it. Prior to `gawk' 3.0, the value of `IGNORECASE' affected regexp operations only. It did not affect string comparison with `==', `!=', and so on. Beginning with version 3.0, both regexp and string comparison operations are also affected by `IGNORECASE'. Beginning with `gawk' 3.0, the equivalences between upper- and lowercase characters are based on the ISO-8859-1 (ISO Latin-1) character set. This character set is a superset of the traditional 128 ASCII characters, that also provides a number of characters suitable for use with European languages. The value of `IGNORECASE' has no effect if `gawk' is in compatibility mode (Note: Command-Line Options.). Case is always significant in compatibility mode. ---------- Footnotes ---------- (1) Experienced C and C++ programmers will note that it is possible, using something like `IGNORECASE = 1 && /foObAr/ { ... }' and `IGNORECASE = 0 || /foobar/ { ... }'. However, this is somewhat obscure and we don't recommend it. automatically generated by info2www version 1.2.2.9 |