fix: better regex support for sed

[nfischer]

Now works with using the empty string as a replacement string

Some details on allowed regexes (in unix sed):

• s/foo/bar/: the string must end in a slash (or with /g or something similar)
• s/foo//: the replacement string is allowed to be empty
• s/a/b/: the regex must be at least one character long
• s/foo/bar\./g: the \. is treated as a literal dot
• s/foo/bar./g: the . is also treated as a literal dot, when in the replacement string
• s/foo./bar/g: the . is not treated literally when in the regex string
• s/foo\//bar\//g: the \/ is treated as a literal / in either string

Fixes #109

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #110 into master will not change coverage

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[nfischer]

@freitagbr could you review?

[freitagbr]

LGTM.