This reference was assembled using information in Regular Expressions. The author, Bruce Barnett, gave permission to use the information.
A regular expression is a set of characters that specify a pattern. The term "regular" comes from a term used to describe grammars and formal languages.
Regular expressions are used when you want to search for specify lines of text containing a particular pattern. Most of the UNIX utilities operate on the files a line at a time. Regular expressions search for patterns on a single line, and not for patterns that start on one line and end on another.
It is simple to search for a specific word or string of characters. Almost every editor on every computer system can do this. Regular expressions are more powerful and flexible. You can search for words of a certain size. You can search for a word with four or more vowels that end with an "s." Numbers, punctuation characters, you name it, a regular expression can find it. What happens once the program you are using find it is another matter. Some just search for the pattern. Others print out the line containing the pattern. Editors can replace the string with a new pattern. It all depends on the utility.
There are three important parts to a regular expression.
1. Anchors are used to specify the position of the pattern in relation to a line of text.
2. Character sets match one or more characters in a single position.
3. Modifiers specify how many times the previous character set is repeated.
A simple example that demonstrates all three parts is the regular expression "^#*." The up arrow is an anchor that indicates the beginning of the line. The character "#" is a simple character set that matches the single character "#." The asterisk is a modifier. In a regular expression it specifies that the previous character set can appear any number of times, including zero. This is a useless regular expression, as you will see shortly.
There are also two types of regular expressions: the "basic" regular expression, and the "extended" regular expression. From now on, if I talk about a "regular expression," it describes a feature in both types.
Most UNIX text facilities are line oriented. Searching for patterns that span several lines is not easy to do. You see, the end of line character is not included in the block of text wthat is searched. It is a separator. Regular expressions examine the text between the separators. If you want to search for a pattern that is at one end or the other, you use anchors. The character "^" is the starting anchor, and the character "$" is the end anchor. The regular expression "^A" will match all lines that start with a capital A. The expression "A$" will match all lines that end with the capital A. If the anchor characters are not used at the proper end of the pattern, then they no longer act as anchors. That is, the "^" is only an anchor if it is the first character in a regular expression. The "$" is only an anchor if it is the last character. The expression "$1" does not have an anchor. Neither is "1^." If you need to match a "^" at the beginning of the line, or a "$" at the end of a line, you must escape the special characters with a backslash. Here is a summary:
Pattern ^A matches "A" at the beginning of a line.
Pattern A$ matches "A" at the end of a line.
Pattern A^ matches "A^" anywhere on a line.
Pattern $A matches "$A" anywhere on a line.
Pattern ^^ matches "^" at the beginning of a line.
Pattern $$ matches "$" at the end of a line.
The use of "^" and "$" as indicators of the beginning or end of a line is a convention other utilities use as well. It is one of those choices that other utilities go along with to maintain consistancy.
The simplest character set is a character. The regular expression "the" contains three character sets: "t," "h" and "e." It will match any line with the string "the" inside it. This would also match the word "other." To prevent this, put spaces before and after the pattern: " the ." You can combine the string with an anchor. The pattern "^From: " will match the lines of a mail message that identify the sender. Some characters have a special meaning in regular expressions. If you want to search for such a character, escape it with a backslash.
The character "." is one of those special meta-characters. By itself it will match any character, except the end-of-line character. The pattern that will match a line with a single characters is
^.$
If you want to match specific characters, you can use the square brackets to identify the exact characters you are searching for. The pattern that will match any line of text that contains exactly one number is
^[0123456789]$
This is verbose. You can use the hyphen between two characters to specify a range:
^[0-9]$
You can intermix explicit characters with character ranges. This pattern will match a single character that is a letter, number, or underscore:
[A-Za-z0-9_]
Character sets can be combined by placing them next to each other. If you wanted to search for a word that
1. Started with a capital letter "T."
2. Was the first word on a line
3. The second letter was a lower case letter
4. Was exactly three letters long, and
5. The third letter was a vowel
the regular expression would be "^T[a-z][aeiou] ."
You can easily search for all characters except those in square brackets by putting a "^" as the first character after the "[." To match all characters except vowels use "[^aeiou]."
Like the anchors in places that can't be considered an anchor, the characters "]" and "-" do not have a special meaning if they directly follow "[." Here are some examples:
Regular expression [] matches the characters "[]".
Regular expression [0] matches the character "0".
Regular expression [0-9] matches any number.
Regular expression [^0-9] matches any character other than a number.
Regular expression [-0-9] matches any number or a "-".
Regular expression [0-9-] matches any number or a "-".
Regular expression [^-0-9] matches any character except a number or a "-".
Regular expression []0-9] matches any number or a "]".
Regular expression [0-9]] matches any number followed by a "]".
Regular expression [0-9-z] matches any number, or any character between "9" and "z".
Regular expression [0-9\-a\]] matches any number, or a "-", a "a", or a "]".
The third part of a regular expression is the modifier. It is used to specify how may times you expect to see the previous character set. The special character "*" matches zero or more copies. That is, the regular expression "0*" matches zero or more zeros, while the expression "[0-9]*" matches zero or more numbers.
This explains why the pattern "^#*" is useless, as it matches any number of "#'s" at the beginning of the line, including zero. Therefore this will match every line, because every line starts with zero or more "#'s."
At first glance, it might seem that starting the count at zero is stupid. Not so. Looking for an unknown number of characters is very important. Suppose you wanted to look for a number at the beginning of a line, and there may or may not be spaces before the number. Just use "^ *" to match zero or more spaces at the beginning of the line. If you need to match one or more, just repeat the character set. That is, "[0-9]*" matches zero or more numbers, and "[0-9][0-9]*" matches one or more numbers.
You can continue the above technique if you want to specify a minimum number of character sets. You cannot specify a maximum number of sets with the "*" modifier. There is a special pattern you can use to specify the minimum and maximum number of repeats. This is done by putting those two numbers between "\{" and "\}." The backslashes deserve a special discussion. Normally a backslash turns off the special meaning for a character. A period is matched by a "\." and an asterisk is matched by a "\*."
If a backslash is placed before a "<," ">," "{," "}," "(," ")," or before a digit, the backslash turns on a special meaning. This was done because these special functions were added later in the life of regular expressions. Changing the meaning of "{" would have broken old expressions.
Having convinced you that "\{" isn't a plot to confuse you, an example is in order. The regular expression to match 4, 5, 6, 7 or 8 lower case letters is
[a-z]\{4,8\}
Any numbers between 0 and 255 can be used. The second number may be omitted, which removes the upper limit. If the comma and the second number are omitted, the pattern must be duplicated the exact number of times specified by the first number.
You must remember that modifiers like "*" and "\{1,5\}" only act as modifiers if they follow a character set. If they were at the beginning of a pattern, they would not be a modifier. Here is a list of examples, and the exceptions:
Regular expression * matches any line with an asterisk.
Regular expression \* matches any line with an asterisk.
Regular expression \\ matches any line with a backslash.
Regular expression ^* matches any line starting with an asterisk.
Regular expression ^A* matches any line.
Regular expression ^A\* matches any line starting with an "A*".
Regular expression ^AA* matches any line if it starts with one "A".
Regular expression ^AA*B matches any line with one or more "A"'s followed by a "B".
Regular expression ^A\{4,8\}B matches any line starting with 4, 5, 6, 7 or 8 "A"'s followed by a "B".
Regular expression ^A\{4,\}B matches any line starting with 4 or more "A"'s followed by a "B".
Regular expression ^A\{4\}B matches any line starting with "AAAAB".
Regular expression \{4,8\} matches any line with "{4,8}".
Regular expression A{4,8} matches any line with "A{4,8}".
Searching for a word isn't quite as simple as it at first appears. The string "the" will match the word "other." You can put spaces before and after the letters and use this regular expression: " the ." However, this does not match words at the beginning or end of the line. And it does not match the case where there is a punctuation mark after the word.
There is an easy solution. The characters "\<" and "\>" are similar to the "^" and "$" anchors, as they don't occupy a position of a character. They do "anchor" the expression between to only match if it is on a word boundary. The pattern to search for the word "the" would be "\<[tT]he\>." The character before the "t" must be either a new line character, or anything except a letter, number, or underscore. The character after the "e" must also be a character other than a number, letter, or underscore or it could be the end of line character.
Another pattern that requires a special mechanism is searching for repeated words. The expression "[a-z][a-z]" will match any two lower case letters. If you wanted to search for lines that had two adjoining identical letters, the above pattern wouldn't help. You need a way of remembering what you found, and seeing if the same pattern occurred again. You can mark part of a pattern using "\(" and "\)." You can recall the remembered pattern with "\" followed by a single digit. Therefore, to search for two identical letters, use "\([a-z]\)\1." You can have 9 different remembered patterns. Each occurrence of "\(" starts a new pattern. The regular expression that would match a 5 letter palindrome, (e.g. "radar"), would be
\([a-z]\)\([a-z]\)[a-z]\2\1
That completes a discussion of the Basic regular expression. Before I discuss the extensions the extended expressions offer, I wanted to mention two potential problem areas.
The other potential point of confusion is the extent of the pattern matches. Regular expressions match the longest possible pattern. That is, the regular expression
A.*B
matches "AAB" as well as "AAAABBBBABCCCCBBBAAAB." This doesn't cause many problems using grep, because an oversight in a regular expression will just match more lines than desired. If you use sed, and your patterns get carried away, you may end up deleting more than you wanted too.
With these regular extensions, those special characters preceded by a backslash no longer have the special meaning: "\{," "\}," "\<," "\>," "\(," "\)" as well as the "\digit." There is a very good reason for this, which I will delay explaining to build up suspense.
The character "?" matches 0 or 1 instances of the character set before, and the character "+" matches one or more copies of the character set. You can't use the \{ and \} in the extended regular expressions, but if you could, you might consider the "?" to be the same as "\{0,1\}" and the "+" to be the same as "\{1,\}."
By now, you are wondering why the extended regular expressions is even worth using. Except for two abbreviations, there are no advantages, and a lot of disadvantages. Therefore, examples would be useful.
The three important characters in the expanded regular expressions are "(," "|," and ")." Together, they let you match a choice of patterns.
Extended expressions don't have the "\<" and "\>" characters. You can compensate by using the alternation mechanism. Matching the word "the" in the beginning, middle, end of a sentence, or end of a line can be done with the extended regular expression:
(^| )the([^a-z]|$)
There are two choices before the word, a space or the beginining of a line. After the word, there must be something besides a lower case letter or else the end of the line. One extra bonus with extended regular expressions is the ability to use the "*," "+," and "?" modifiers after a "(...)" grouping. The following will match "a simple problem," "an easy problem," as well as "a problem."
egrep "a[n]? (simple|easy)? problem" data
I promised to explain why the backslash characters don't work in extended regular expressions. Well, perhaps the "\{...\}" and "\<...\>" could be added to the extended expressions. These are the newest addition to the regular expression family. They could be added, but this might confuse people if those characters are added and the "\(...\)" are not. And there is no way to add that functionality to the extended expressions without changing the current usage. Do you see why? It's quite simple. If "(" has a special meaning, then "\(" must be the ordinary character. This is the opposite of the Basic regular expressions, where "(" is ordinary, and "\(" is special. The usage of the parentheses is incompatable, and any change could break old programs.
If the extended expression used "( ..|...)" as regular characters, and "\(...\|...\)" for specifying alternate patterns, then it is possible to have one set of regular expressions that has full functionality.