Regular Expressions

Regular expression are a lot more powerful though. We can search for lines containing the word 'file' not preceeded by 'old'. We can search for empty lines, lines which contain one word or another. To be able to do this it is necessary to understand the syntax and grammar of regular expressions. A regular expression is always tried to be matched against some text. If a match succeeds grep will print that line.

• A '.' can stand for any character
• A list of characters enclosed by '[' and ']' matches any single character in that list. If the first character in the list is a '^ ', then it matches any single character not in the list.
• The '^ ' matches the beginning of a line and the '$' matches the end of a line.
• To match one or another regular expression,both regular expressions can be bracketed with '(' and ')' and separated by '|'.
• A regular expression is optional and matched at most once, if it is followed by a '?'.
• A regular expression is matched zero or many times if is followed by a '*'.
• A regular expression is matched one or many times if is followed by a '+'.
• A regular expression is matched n times if is followed by a '{n}'.
• A regular expression is matched n or more times if is followed by a '{n,}'.
• A regular expression is matched m or less times if is followed by a '{,m}'.
• A regular expression is matched at least n times and at most m times if is followed by a '{n,m}'.

In some versions of grep the following characters have to be preceeded by a '\': ?, +, {, |, (, ).

• Examples