Regex replacement with lowercase monthnames for biblatex/biber to eliminate the warning “month not integer…”

Question

I had already asked a nearly identical question here

I am using biblatex with biber from TL2018 for my PhD thesis. Due to various reasons, I am forced to use Mendeley for citation managements and cannot switch to another tool now. Mendeley is capable of generating a bib file - but it is compatible with only the classic natbib/bibtex combo (although it correctly escapes special characters).

I understand that with biblatex/biber combo, the month field is not a string, but rather must be an integer. So, as expected and discussed here, I get the warning

WARN - month field 'jan' in entry 'Guo2012' is not an integer - this will probably not sort properly.

(this time for jan wherein unlike my original question, the month does not begin with captial J).

For tackling the original question, I used the regex given in that question in my preamble (given below)

\DeclareSourcemap{
    \maps[datatype=bibtex]{
        \map[overwrite]{
            \step[fieldsource=month, match=\regexp{\Ajan\Z}, replace=1]
            \step[fieldsource=month, match=\regexp{\Afeb\Z}, replace=2]
            \step[fieldsource=month, match=\regexp{\Amar\Z}, replace=3]
            \step[fieldsource=month, match=\regexp{\Aapr\Z}, replace=4]
            \step[fieldsource=month, match=\regexp{\Amay\Z}, replace=5]
            \step[fieldsource=month, match=\regexp{\Ajun\Z}, replace=6]
            \step[fieldsource=month, match=\regexp{\Ajul\Z}, replace=7]
            \step[fieldsource=month, match=\regexp{\Aaug\Z}, replace=8]
            \step[fieldsource=month, match=\regexp{\Asep\Z}, replace=9]
            \step[fieldsource=month, match=\regexp{\Aoct\Z}, replace=10]
            \step[fieldsource=month, match=\regexp{\Anov\Z}, replace=11]
            \step[fieldsource=month, match=\regexp{\Adec\Z}, replace=12]
        }
    }
}

How can this be modified suitably to handle this lowercase situation?