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You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 in the result string matches the bracketed reference in the pattern match

sed -r 's#^(.{30})#\1213 #' file

In your updated question you now say there are 878 characters before the insert. So just amend the example's 30 to reality's 878 and insert XXX

sed -r 's#^(.{878})#\1XXX#' file

The same process can apply for any fixed-width modification.

You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 in the result string matches the bracketed reference in the pattern match

sed -r 's#^(.{30})#\1213 #' file

In your updated question you now say there are 878 characters before the insert. So just amend the example's 30 to reality's 878 and insert XXX

sed -r 's#^(.{878})#\1XXX#' file

The same process can apply for any fixed-width modification.

You can use perl too, which doesn't have the line length limitations that plague some implementations of sed,

perl -pe 's#^(.{878})#$1XXX#' file

You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 in the result string matches the bracketed reference in the pattern match

sed -r 's#^(.{30})#\1213 #' file

In your updated question you now say there are 878 characters before the insert. So just amend the example's 30 to reality's 878

sed -r 's#^(.{878})#\1213 #' file

The same process can apply for any fixed-width modification.

You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 in the result string matches the bracketed reference in the pattern match

sed -r 's#^(.{30})#\1213 #' file

In your updated question you now say there are 878 characters before the insert. So just amend the example's 30 to reality's 878 and insert XXX

sed -r 's#^(.{878})#\1XXX#' file

The same process can apply for any fixed-width modification.

You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 and \2 in the result string match the bracketed references in the pattern match

sed -r 's#^(.{30})(.*)#\1213 \2#' file

You can use a counted RE. For example, x{12} will match 12 x characters, and y{1,3} would match 1, 2, or 3 y characters. Here we're going to use .{30} to match 30 character wildcards (i.e. 30 of any character). The \1 in the result string matches the bracketed reference in the pattern match

sed -r 's#^(.{30})#\1213 #' file

In your updated question you now say there are 878 characters before the insert. So just amend the example's 30 to reality's 878

sed -r 's#^(.{878})#\1213 #' file

The same process can apply for any fixed-width modification.