Why can't I get the output of grep?

when I was in regular grep, I couldn"t get its output. How to solve this situation:

echo "AAA<%=CCC=%>BBB" | grep -Po "<%=(.+?)=%>"
var=$(echo "AAA<%=CCC=%>BBB" | grep -Po "<%=(.+?)=%>")
echo $var
Mar.29,2021

this problem does not exist, but is caused by the use of mingw64, in the test environment. Thank you @ Feng_Yu for your kind reply.

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b378ad-2c096.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b378ad-2c096.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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