Correct way to match a leading space with sed (all of them)?

Question

How to match a leading space with sed (all of them)? I'm not talking about leading tabs, but rather only on leading spaces.

From a small test I did in Nano this seems to be correct:

sed "s/^ //g"

Do you find something wrong with this method?

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_{Note: "All of them" means all leading spaces in the document, in case there are 2 or more, and not just one.}

Solution

Remove leading spaces: sed "s/^ *//"

Remove leading whitespace: sed "s/^[[:space:]]*//"

Remove leading spaces and tabs: sed "s/^[ \t]*//" (works in GNU sed) or
sed 's/^[[:blank:]]*//' (works with any sed) or sed $'s/^[ \t]*//' (in ksh/Bash/etc. to give a literal tab to sed)

As said in the comments, the /g specifier does nothing, as the beginning of line appears only once in the line, and even /g does not retry the pattern more than one. You'd need to add a conditional branch explicitly to repeat the substitution: sed -e :a -e 's/^ //' -e ta

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^ * matches the empty string (no spaces) too, but that doesn't matter here. If you want to match lines that have at least one space, use ^ * (double space) or ^ + in extended regex. E.g. to change all indentations to exactly two spaces, use sed -e 's/^ */ /' or sed -Ee 's/^ +/ /' (-E is supported in e.g. GNU and FreeBSD)

Comments

[RomanPerekhrest]

tab is also IN whitespace category. sed "s/^[[:space:]]//g"

[user9303970]

@RomanPerekhrest So I should go through a paradigm change --- from now and long I shall say "space" for a simple space as with the SPACE key in my keboard and "whitespace" for any space in the general sense (space or tabulation), correct?

[Stéphane Chazelas]

Note that sed "s/^[ \t]*//" works in GNU sed only if POSIXLY_CORRECT is not in the environment. Otherwise it strips ts and backslashes like other sed implementations.

[Oscar Zhang]

I wonder what's difference btw spaces and whitespace by definition and why are they treated differently? In some case, sed "s/^ *//" just works if I have one space, while in other case, say I extract the righthand side of a diff output(after |<>), I have to use 2nd one sed "s/^[[:space:]]*//" to do the job.

[ilkkachu]

@OscarZhang, tabs and newlines are "whitespace" too, in addition to the space I get from the big wide button. sed "s/^ *//" removes just spaces, not tabs etc. [[:blank:]] would match spaces and tabs and [[:space:]] matches some other stuff too, even newlines (not that you'd see that easily with sed or grep)