Adam Clarke on Jeremiah 37:5

Then Pharaoh's army - This was Pharaoh-hophra or Apries, who then reigned in Egypt in place of his father Necho. See Eze 29:6, etc. Nebuchadnezzar, hearing that the Egyptian army, on which the Jews so much depended, was on their march to relieve the city, suddenly raised the siege, and went to meet them. In the interim Zedekiah sent to Jeremiah to inquire of the Lord to know whether they might consider themselves in safety.