regex - Replace all whitespace with a line break/paragraph mark to make a word list

Question

I am trying to vocab list for a Greek text we are translating in class. I want to replace every space or tab character with a paragraph mark so that every word appears on its own line. Can anyone give me the sed command, and explain what it is that I'm doing? I’m still trying to figure sed out.

Answer 1

For reasonably modern versions of sed, edit the standard input to yield the standard output with

$ echo 'τ?χνη βιβλ?ο γη κ?πο?' | sed -E -e 's/[[:blank:]]+/
/g'
τ?χνη
βιβλ?ο
γη
κ?πο?

If your vocabulary words are in files named lesson1 and lesson2, redirect sed’s standard output to the file all-vocab with

sed -E -e 's/[[:blank:]]+/
/g' lesson1 lesson2 > all-vocab

What it means:

• The character class [[:blank:]] matches either a single space character or a single tab character.
  • Use [[:space:]] instead to match any single whitespace character (commonly space, tab, newline, carriage return, form-feed, and vertical tab).
  • The + quantifier means match one or more of the previous pattern.
  • So [[:blank:]]+ is a sequence of one or more characters that are all space or tab.
• The in the replacement is the newline that you want.
• The /g modifier on the end means perform the substitution as many times as possible rather than just once.
• The -E option tells sed to use POSIX extended regex syntax and in particular for this case the + quantifier. Without -E, your sed command becomes sed -e 's/[[:blank:]]+/ /g'. (Note the use of + rather than simple +.)

Perl Compatible Regexes

For those familiar with Perl-compatible regexes and a PCRE-capable sed, use s+ to match runs of at least one whitespace character, as in

sed -E -e 's/s+/
/g' old > new

or

sed -e 's/s+/
/g' old > new

These commands read input from the file old and write the result to a file named new in the current directory.

Maximum portability, maximum cruftiness

Going back to almost any version of sed since Version 7 Unix, the command invocation is a bit more baroque.

$ echo 'τ?χνη βιβλ?ο γη κ?πο?' | sed -e 's/[ ][ ]*/
/g'
τ?χνη
βιβλ?ο
γη
κ?πο?

Notes:

• Here we do not even assume the existence of the humble + quantifier and simulate it with a single space-or-tab ([ ]) followed by zero or more of them ([ ]*).
• Similarly, assuming sed does not understand for newline, we have to include it on the command line verbatim.
  • The and the end of the first line of the command is a continuation marker that escapes the immediately following newline, and the remainder of the command is on the next line.
    • Note: There must be no whitespace preceding the escaped newline. That is, the end of the first line must be exactly backslash followed by end-of-line.
  • This error prone process helps one appreciate why the world moved to visible characters, and you will want to exercise some care in trying out the command with copy-and-paste.

Note on backslashes and quoting

The commands above all used single quotes ('') rather than double quotes (""). Consider:

$ echo '\\' ""
\\ \

That is, the shell applies different escaping rules to single-quoted strings as compared with double-quoted strings. You typically want to protect all the backslashes common in regexes with single quotes.