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gawk: Case-sensitivity

 3.8 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; ⇒String 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 (⇒Built-in Variables).  When
 'IGNORECASE' is not zero, _all_ regexp and string operations ignore
    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, as there is no
 straightforward way to set 'IGNORECASE' just for the pattern of a
 particular rule.(1)  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
 (⇒Other Arguments; also ⇒Using BEGIN/END).  Setting
 'IGNORECASE' from the command line is a way to make a program case
 insensitive without having to edit it.
    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.(2)
    The value of 'IGNORECASE' has no effect if 'gawk' is in compatibility
 mode (⇒Options).  Case is always significant in compatibility
    ---------- 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.
    (2) If you don't understand this, don't worry about it; it just means
 that 'gawk' does the right thing.
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