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gawk: Ranges and Locales

 
 A.8 Regexp Ranges and Locales: A Long Sad Story
 ===============================================
 
 This minor node describes the confusing history of ranges within regular
 expressions and their interactions with locales, and how this affected
 different versions of 'gawk'.
 
    The original Unix tools that worked with regular expressions defined
 character ranges (such as '[a-z]') to match any character between the
 first character in the range and the last character in the range,
 inclusive.  Ordering was based on the numeric value of each character in
 the machine's native character set.  Thus, on ASCII-based systems,
 '[a-z]' matched all the lowercase letters, and only the lowercase
 letters, as the numeric values for the letters from 'a' through 'z' were
 contiguous.  (On an EBCDIC system, the range '[a-z]' includes additional
 nonalphabetic characters as well.)
 
    Almost all introductory Unix literature explained range expressions
 as working in this fashion, and in particular, would teach that the
 "correct" way to match lowercase letters was with '[a-z]', and that
 '[A-Z]' was the "correct" way to match uppercase letters.  And indeed,
 this was true.(1)
 
    The 1992 POSIX standard introduced the idea of locales (⇒
 Locales).  Because many locales include other letters besides the
 plain 26 letters of the English alphabet, the POSIX standard added
 character classes (⇒Bracket Expressions) as a way to match
 different kinds of characters besides the traditional ones in the ASCII
 character set.
 
    However, the standard _changed_ the interpretation of range
 expressions.  In the '"C"' and '"POSIX"' locales, a range expression
 like '[a-dx-z]' is still equivalent to '[abcdxyz]', as in ASCII. But
 outside those locales, the ordering was defined to be based on
 "collation order".
 
    What does that mean?  In many locales, 'A' and 'a' are both less than
 'B'.  In other words, these locales sort characters in dictionary order,
 and '[a-dx-z]' is typically not equivalent to '[abcdxyz]'; instead, it
 might be equivalent to '[ABCXYabcdxyz]', for example.
 
    This point needs to be emphasized: much literature teaches that you
 should use '[a-z]' to match a lowercase character.  But on systems with
 non-ASCII locales, this also matches all of the uppercase characters
 except 'A' or 'Z'!  This was a continuous cause of confusion, even well
 into the twenty-first century.
 
    To demonstrate these issues, the following example uses the 'sub()'
 function, which does text replacement (⇒String Functions).  Here,
 the intent is to remove trailing uppercase characters:
 
      $ echo something1234abc | gawk-3.1.8 '{ sub("[A-Z]*$", ""); print }'
      -| something1234a
 
 This output is unexpected, as the 'bc' at the end of 'something1234abc'
 should not normally match '[A-Z]*'.  This result is due to the locale
 setting (and thus you may not see it on your system).
 
    Similar considerations apply to other ranges.  For example, '["-/]'
 is perfectly valid in ASCII, but is not valid in many Unicode locales,
 such as 'en_US.UTF-8'.
 
    Early versions of 'gawk' used regexp matching code that was not
 locale-aware, so ranges had their traditional interpretation.
 
    When 'gawk' switched to using locale-aware regexp matchers, the
 problems began; especially as both GNU/Linux and commercial Unix vendors
 started implementing non-ASCII locales, _and making them the default_.
 Perhaps the most frequently asked question became something like, "Why
 does '[A-Z]' match lowercase letters?!?"
 
    This situation existed for close to 10 years, if not more, and the
 'gawk' maintainer grew weary of trying to explain that 'gawk' was being
 nicely standards-compliant, and that the issue was in the user's locale.
 During the development of version 4.0, he modified 'gawk' to always
 treat ranges in the original, pre-POSIX fashion, unless '--posix' was
 used (⇒Options).(2)
 
    Fortunately, shortly before the final release of 'gawk' 4.0, the
 maintainer learned that the 2008 standard had changed the definition of
 ranges, such that outside the '"C"' and '"POSIX"' locales, the meaning
 of range expressions was _undefined_.(3)
 
    By using this lovely technical term, the standard gives license to
 implementers to implement ranges in whatever way they choose.  The
 'gawk' maintainer chose to apply the pre-POSIX meaning both with the
 default regexp matching and when '--traditional' or '--posix' are used.
 In all cases 'gawk' remains POSIX-compliant.
 
    ---------- Footnotes ----------
 
    (1) And Life was good.
 
    (2) And thus was born the Campaign for Rational Range Interpretation
 (or RRI). A number of GNU tools have already implemented this change, or
 will soon.  Thanks to Karl Berry for coining the phrase "Rational Range
 Interpretation."
 
    (3) See the standard
 (http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05)
 and its rationale
 (http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap09.html#tag_21_09_03_05).
 
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