6.3.2.3 String Comparison Based on Locale Collating Order

The POSIX standard used to say that all string comparisons are performed based on the locale’s collating order. This is the order in which characters sort, as defined by the locale (for more discussion, see section Where You Are Makes a Difference). This order is usually very different from the results obtained when doing straight byte-by-byte comparison.³⁴

Because this behavior differs considerably from existing practice, gawk only implemented it when in POSIX mode (see section Command-Line Options). Here is an example to illustrate the difference, in an en_US.UTF-8 locale:

$ gawk 'BEGIN { printf("ABC < abc = %s\n",
>                     ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-| ABC < abc = TRUE
$ gawk --posix 'BEGIN { printf("ABC < abc = %s\n",
>                             ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-| ABC < abc = FALSE

Fortunately, as of August 2016, comparison based on locale collating order is no longer required for the == and != operators.³⁵ However, comparison based on locales is still required for <, <=, >, and >=. POSIX thus recommends as follows:

Since the == operator checks whether strings are identical, not whether they collate equally, applications needing to check whether strings collate equally can use:

a <= b && a >= b

As of version 4.2, gawk continues to use locale collating order for <, <=, >, and >= only in POSIX mode.