Each will have a number before it. A regular expression is a pattern that text can either match or not match. A regular expression is given as a string of characters. Most of these have no special meaning, i.e., they match only themselves. So, a backslash has a special meaning, too.
Many programs use regular expressions.
It prints all lines of the file that contain text matching the regular expression. Shell Scripts A shell script is a text file of shell commands, which can be executed just as if you typed them in. The shell knows about the file. When you log in, csh looks around for executable files. Scripts that can be executed by typing their name are a bit tricky to write.