Is there any way to remove the word dynamically?

Is there any way to remove the word dynamically?

I am using below code to remove word "USER1". on my richtextbox1.
Is there any way to remove the word dynamically?
For example on my richtextbox1, I have below sentence and what ever user
put a word inside my textbox1 "USER1". will be replaced with new input, so
pleace that USER1 is holding on below sentence will be replaced with new
input, so I need something to dynamically to remove the word.
How can I add my textbox1 inside below code to delete user input word
dynamically?
Regex reg = new Regex("(ALTER TABLE .+ REFERENCES\\s+)\"USER1\"[.](.+)");
richTextBox1.Text = reg.Replace(richTextBox1.Text, "$1$2");
After using my code my sentence will be; but USER1 is depends on the user
input and it can be change.
ALTER TABLE "GRADE" ADD CONSTRAINT "GR_ENR_FK" FOREIGN KEY
("STUDENT_ID","SECTION_ID") REFERENCES
"ENROLLMENT"("STUDENT_ID","SECTION_ID") ENABLE;