From Erlang Community

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[edit] Problem

You have a string made up of multiple lines, and you need to build a string, adding or removing spaces so that all lines are indented equally.

[edit] Solution

reindent(Str, Num_spaces) ->
    Lmargin = string:chars($ , Num_spaces),
    Str_set = string:tokens(Str, "\n"),
    lists:flatmap(
        fun(X) ->
            Lmargin ++ string:strip(X,left) ++ "\n"
        end, Str_set).

The following examples illustrate the reindention of text, including cases where more space is already provided on the line than desired:

1> io:fwrite("~s", [reindent("one\ntwo\nthree", 3)]).
   one
   two
   three
2> io:fwrite("~s", [reindent("one\n      two\n   three", 3)]).
   one
   two
   three

This definition uses string:tokens to split the string argument on newliness, then uses lists:flatmap to process each line, removing any existing leading space, prepending the proper indentation to the start of each line, and appending the newline (lost during the tokenization step).