Question

How to transform with perl this multiline:

Colours:
Red
Green
Yellow
Blue

In:

Colours: Red
Green
Yellow
Blue

Answer 1

The Unicode linebreak grapheme is \R, and is supported from v5.10 onwards.

You could therefore change the first linebreak to a space in the whole file this way:

$ perl -Mv5.10 -CSD -i.orig -0777 -pe 's/\R/ /' some_utf8_file.txt

There are other ways that aren’t so wasteful of memory, but they can be tricky to get right. You might omit the -0777 and see whether that is good enough for you in this case.

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EDIT: Regex Escapes

Here are the escapes supported in regexes, including what release it was first supported in, with release numbers rounded toward even beginning with v5.6.

Release  Rx Escape   Meaning
=======  ==========  ===========================================================================================
v1.0     \0          Match character number zero (U+0000, NULL, NUL).
v1.0     \0N,\0NN    Match octal character up to octal 077.
v1.0     \N          Match Nth capture group (decimal) if not in charclass and that many seen, else (octal) character up to octal 377.
v1.0     \NN         Match Nth capture group (decimal) if not in charclass and that many seen, else (octal) character up to octal 377.
v1.0     \NNN        Match Nth capture group (decimal) if not in charclass and that many seen, else (octal) character up to octal 377.
v4.0     \a          Match the alert character (ALERT, BEL).
v5.0     \A          True at the beginning of a string only, not in charclass.
v1.0     \b          Match the backspace char (BACKSPACE, BS) in charclass only.
v1.0     \b          True at Unicode word boundary, outside of charclass only.
v1.0     \B          True when not at Unicode word boundary, not in charclass.
v4.0     \cX         Match ASCII control character Control-X (\cZ, \c[, \c?, etc).
v5.6     \C          Match one byte (C char) even in UTF‑8 (dangerous!), not in charclass.
v1.0     \d          Match any Unicode digit character.
v1.0     \D          Match any Unicode nondigit character.
v4.0     \e          Match the escape character (ESCAPE, ESC, not backslash).
v4.0     \E          End case (\F, \L, \U) or quotemeta (\Q) translation, only if interpolated.
v1.0     \f          Match the form feed character (FORM FEED, FF).
v5.16    \F          Foldcase (not lowercase) till \E, only if interpolated.
v5.10    \g{GROUP}   Match the named or numbered capture group, not in charclass.
v5.0     \G          True at end-of-match position of prior m//g or pos() setting, not in charclass.
v5.10    \h          Match any Unicode horizontal whitespace character.
v5.10    \H          Match any Unicode character except horizontal whitespace.
v5.10    \k<GROUP>   Match the named capture group; also \k'NAME', not in charclass.
v5.10    \K          Keep text to the left of \K out of match, not in charclass.
v4.0     \l          Lowercase (not foldcase) next character only, only if interpolated.
v4.0     \L          Lowercase (not foldcase) till \E., only if interpolated.
v1.0     \n          Match the newline character (usually LINE FEED, LF).
v5.12    \N          Match any character except newline.
v5.6     \N{NAME}    Match the named character or named alias, or if outside of charclass named sequence, but only if interpolated and charnames loaded.
v5.14    \o{NNNNNN}  Match the character given in any number of octal digits.
v5.6     \p{PROP}    Match any character with the named property.
v5.6     \P{PROP}    Match any character without the named property.
v4.0     \Q          Quote (de-meta) metacharacters till \E.
v1.0     \r          Match the return character (usually CARRIAGE RETURN, CR).
v5.10    \R          Match any Unicode linebreak grapheme, only outside of charclass.
v1.0     \s          Match any Unicode whitespace character except \cK.
v1.0     \S          Match any Unicode nonwhitespace character or \cK.
v1.0     \t          Match the tab character (CHARACTER TABULATION, HT).
v4.0     \u          Titlecase (not uppercase) next character only, only if interpolated.
v4.0     \U          Uppercase (not titlecase) till \E, only if interpolated.
v5.10    \v          Match any Unicode vertical whitespace character.
v5.10    \V          Match any character except Unicode vertical whitespace.
v1.0     \w          Match any Unicode “word” character (alphabetics, digits, combining marks, and connector punctuation)
v1.0     \W          Match any Unicode nonword character.
v4.0     \xH         Match the character given in one hex digit.
v4.0     \xHH        Match the character given in two hex digits.
v5.6     \x{HHHHHH}  Match the character given in any number of hex.
v5.6     \X          Match Unicode extended grapheme cluster, only outside of charclass.
v5.5     \z          True at end of string only.
v5.0     \Z          True right before optional final newline.