diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index dce8c15126..27bbe60263 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -2100,9 +2100,9 @@ module Net
 
       BEG_REGEXP = /\G(?:\
 (?# 1:  SPACE   )( +)|\
-(?# 2:  NIL     )(NIL)(?=[\x80-\xff(){ \x00-\x1f\x7f%*#{'"'}\\\[\]+])|\
-(?# 3:  NUMBER  )(\d+)(?=[\x80-\xff(){ \x00-\x1f\x7f%*#{'"'}\\\[\]+])|\
-(?# 4:  ATOM    )([^\x80-\xff(){ \x00-\x1f\x7f%*#{'"'}\\\[\]+]+)|\
+(?# 2:  NIL     )(NIL)(?=[\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+])|\
+(?# 3:  NUMBER  )(\d+)(?=[\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+])|\
+(?# 4:  ATOM    )([^\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+]+)|\
 (?# 5:  QUOTED  )"((?:[^\x00\r\n"\\]|\\["\\])*)"|\
 (?# 6:  LPAR    )(\()|\
 (?# 7:  RPAR    )(\))|\