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Accepted
Mario Palumbo
Asked: 2020-09-26 02:56:09 +0800 CST

"sed" to add a space only under a certain condition

  • 772

I have to add a space after the # to every occurrence, only if the # is at the beginning of the line and after the # there is at least one character that is not the space. For example this code:

echo "# ok" | sed "s|^#[^ ]|# |g"

Returns # ok as expected, but this code:

echo "#ok" | sed "s|^#[^ ]|# |g"

Returns # k and not # ok as expected.
How do I get the # ok output?

Edit:

This is the code that solves my problem, thanks to @FedonKadifeli:

echo -e "#ok\n# ok\n #ok\n#ok #ok\n##ok #ok"

Returns:

#ok
# ok
 #ok
#ok #ok
##ok #ok

This code:

echo -e "#ok\n# ok\n #ok\n#ok #ok\n##ok #ok" | sed -r 's|^#(#*)([^[:space:]#])|#\1 \2|g'

Returns:

# ok
# ok
 #ok
# ok #ok
## ok #ok
command-line sed text-processing

3 Answers

  1. Best Answer

    One fairly straightforward approach would be to replace the first "hash" by "hash space" only in lines that begin with "hash not space":

    $ echo "#ok" | sed "/^#[^ ]/s|#|# |"
# ok

    In regex variants that provide it, like Perl, you could use negative lookahead:

    $ echo "#ok" | perl -pe 's/^#(?! )/# /'
# ok
    • 6

  2. You can use sed "back-reference" like this:

    sed -r 's/^#([^ ])/# \1/'
    • 5

  3. This is the code that solves my problem, thanks to @steeldriver and @FedonKadifeli:

    echo -e "#ok\n#ok\n# ok\n# ok\n#ok\nnobegin #ok"

    Returns:

    #ok
#ok
# ok
# ok
#ok
nobegin #ok

    This code:

    echo -e "#ok\n#ok\n# ok\n# ok\n#ok\nnobegin #ok" | sed "\|^#[^[:space:]]|s|#|# |g"

    Or this more logic code:

    echo -e "#ok\n#ok\n# ok\n# ok\n#ok\nnobegin #ok" | sed -r "s|^#([^[:space:]])|# \1|g"

    Returns:

    # ok
# ok
# ok
# ok
# ok
nobegin #ok
    • 0