Libx GNU sed User Guide

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    5.9 Multibyte characters and Locale Considerations

    GNU sed processes valid multibyte characters in multibyte locales (e.g. UTF-8). 7

    The following example uses the Greek letter Capital Sigma (Σ, Unicode code point 0x03A3). In a UTF-8 locale, sed correctly processes the Sigma as one character despite it being 2 octets (bytes): 

    $ locale | grep LANG
LANG=en_US.UTF-8

$ printf 'a\u03A3b'
aΣb

$ printf 'a\u03A3b' | sed 's/./X/g'
XXX

$ printf 'a\u03A3b' | od -tx1 -An
 61 ce a3 62

    To force sed to process octets separately, use the C locale (also known as the POSIX locale): 

    $ printf 'a\u03A3b' | LC_ALL=C sed 's/./X/g'
XXXX
    (7)

    Some regexp edge-cases depends on the operating system and libc implementation. The examples shown are known to work as-expected on GNU/Linux systems using glibc.