Or use rename
$ touch foo{1..5}.txt
$ rename -v 's/foo/bar/' foo*
foo1.txt renamed as bar1.txt
foo2.txt renamed as bar2.txt
foo3.txt renamed as bar3.txt
foo4.txt renamed as bar4.txt
foo5.txt renamed as bar5.txt
$ rename -v 's/\.txt/.text/' *.txt
bar1.txt renamed as bar1.text
bar2.txt renamed as bar2.text
bar3.txt renamed as bar3.text
bar4.txt renamed as bar4.text
bar5.txt renamed as bar5.text
$ rename -v 's/(.*).text/1234-$1.txt/' *.text
bar1.text renamed as 1234-bar1.txt
bar2.text renamed as 1234-bar2.txt
bar3.text renamed as 1234-bar3.txt
bar4.text renamed as 1234-bar4.txt
bar5.text renamed as 1234-bar5.txt