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Post#4 requires the match at the beginning of the string.
Post#2 does only one substitution and also does it for A.B.
The following requires a digit after the dot, and because it is part of the match must mark it in \( \) and put it back via a back-reference. Also it does the substitution globally.
echo '|*|.5|*|0.2|*|A.B|*|' | sed 's/|\(\.[0-9]\)/|0\1/g'