5. Regular Expressions

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions. grep understands two different versions of regular expression syntax: "basic"(BRE) and "extended"(ERE). In GNU grep, there is no difference in available functionality using either syntax. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of several repetition operators:

` .'

The period `.' matches any single character.

` ?'

The preceding item is optional and will be matched at most once.

` *'

The preceding item will be matched zero or more times.

` +'

The preceding item will be matched one or more times.

` {n}'

The preceding item is matched exactly n times.

` {n,}'

The preceding item is matched n or more times.

` {n,m}'

The preceding item is matched at least n times, but not more than m times.

Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.

Two regular expressions may be joined by the infix operator `|'; the resulting regular expression matches any string matching either subexpression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression may be enclosed in parentheses to override these precedence rules.

5.1 Character Class

A bracket expression is a list of characters enclosed by `[' and `]'. It matches any single character in that list; if the first character of the list is the caret `^', then it matches any character not in the list. For example, the regular expression `[0123456789]' matches any single digit.

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive, using the locale's collating sequence and character set. For example, in the default C locale, `[a-d]' is equivalent to `[abcd]'. Many locales sort characters in dictionary order, and in these locales `[a-d]' is typically not equivalent to `[abcd]'; it might be equivalent to `[aBbCcDd]', for example. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value `C'.

Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their interpretation depends on the LC_CTYPE locale; the interpretation below is that of the C locale, which is the default if no LC_CTYPE locale is specified.

` [:alnum:]'

Alphanumeric characters: `[:alpha:]' and `[:digit:]'.

` [:alpha:]'

Alphabetic characters: `[:lower:]' and `[:upper:]'.

` [:blank:]'

Blank characters: space and tab.

` [:cntrl:]'

Control characters. In ASCII, these characters have octal codes 000 through 037, and 177 (DEL). In other character sets, these are the equivalent characters, if any.

` [:digit:]'

Digits: 0 1 2 3 4 5 6 7 8 9.

` [:graph:]'

Graphical characters: `[:alnum:]' and `[:punct:]'.

` [:lower:]'

Lower-case letters: a b c d e f g h i j k l m n o p q r s t u v w x y z.

` [:print:]'

Printable characters: `[:alnum:]', `[:punct:]', and space.

` [:punct:]'

Punctuation characters: ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.

` [:space:]'

Space characters: tab, newline, vertical tab, form feed, carriage return, and space.

` [:upper:]'

Upper-case letters: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.

` [:xdigit:]'

Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.

For example, `[[:alnum:]]' means `[0-9A-Za-z]', except the latter depends upon the C locale and the ASCII character encoding, whereas the former is independent of locale and character set. (Note that the brackets in these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket list.)

Most metacharacters lose their special meaning inside lists.

` ]'

ends the list if it's not the first list item. So, if you want to make the `]' character a list item, you must put it first.

` [.'

represents the open collating symbol.

` .]'

represents the close collating symbol.

` [='

represents the open equivalence class.

` =]'

represents the close equivalence class.

` [:'

represents the open character class followed by a valid character class name.

` :]'

represents the close character class followed by a valid character class name.

` -'

represents the range if it's not first or last in a list or the ending point of a range.

` ^'

represents the characters not in the list. If you want to make the `^' character a list item, place it anywhere but first.

5.2 Backslash Character

The `\' when followed by certain ordinary characters take a special meaning :

` `\b''

Match the empty string at the edge of a word.

` `\B''

Match the empty string provided it's not at the edge of a word.

` `\<''

Match the empty string at the beginning of word.

` `\>''

Match the empty string at the end of word.

` `\w''

Match word constituent, it is a synonym for `[[:alnum:]]'.

` `\W''

Match non word constituent, it is a synonym for `[^[:alnum:]]'.

For example , `\brat\b' matches the separate word `rat', `c\Brat\Be' matches `crate', but `dirty \Brat' doesn't match `dirty rat'.

5.3 Anchoring

The caret `^' and the dollar sign `$' are metacharacters that respectively match the empty string at the beginning and end of a line.

5.4 Back-reference

The back-reference `\n', where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression. For example, `(a)\1' matches `aa'. When use with alternation if the group does not participate in the match, then the back-reference makes the whole match fail. For example, `a(.)|b\1' will not match `ba'. When multiple regular expressions are given with `-e' or from a file `-f file', the back-referecences are local to each expression.

5.5 Basic vs Extended

In basic regular expressions the metacharacters `?', `+', `{', `|', `(', and `)' lose their special meaning; instead use the backslashed versions `\?', `\+', `\{', `\|', `\(', and `\)'.

Traditional egrep did not support the `{' metacharacter, and some egrep implementations support `\{' instead, so portable scripts should avoid `{' in `egrep' patterns and should use `[{]' to match a literal `{'.

GNU egrep attempts to support traditional usage by assuming that `{' is not special if it would be the start of an invalid interval specification. For example, the shell command `egrep '{1'' searches for the two-character string `{1' instead of reporting a syntax error in the regular expression. POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

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