Regular expressions documentation while using grep - regex

Question

I am trying to find some comprehensive documentation on character classes in regular expressions that could be used while using grep. I tried
info grep
man grep
man 7 regex
but could not find all the characters classes listed down in the documentation.
I am looking for some comprehensive documentation on regex that grep uses. Is there such a documentation available?

Answer 1

grep has three options for regex -E or --extended-regexp -G or --basic-regexp and -P or --perl-regexp.
Extended / Basic Regex Classes: Follow POSIX Classes
Perl Regex Classes: Follow Perl Classes
From the command line POSIX regex information can be accessed via man 7 regex where as Perl regex data can be accessed via perldoc perlre

Answer 2

http://linux.about.com/od/commands/l/blcmdl1_grep.htm
Finally, certain named classes of characters are predefined within
bracket expressions, as follows. Their names are self explanatory, and
they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:],
[:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:].
For example, [[:alnum:]] means [0-9A-Za-z], except the latter form
depends upon the C locale and the ASCII character encoding, whereas
the former is independent of locale and character set.

Answer 3

When I do man grep, this is what I get:
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" and "extended." 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 meta-character with
special meaning may be quoted by preceding it with a backslash.
The period . matches any single character.
Character Classes and Bracket Expressions
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
names are self explanatory, and they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:],
[:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, [[:alnum:]] means [0-9A-Za-z],
except the latter form 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 expression.) Most
meta-characters lose their special meaning inside bracket expressions. To include a literal ] place it
first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a
literal - place it last.
Anchoring
The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the
beginning and end of a line.
The Backslash Character and Special Expressions
The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b
matches the empty string at the edge of a word, and \B matches the empty string provided it's not at the
edge of a word. The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum:]].
Repetition
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and 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.
{,m} The preceding item is matched at most m times.
{n,m} The preceding item is matched at least n times, but not more than m times.
Concatenation
Two regular expressions may be concatenated; the resulting regular expression matches any string formed by
concatenating two substrings that respectively match the concatenated expressions.
Alternation
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any
string matching either alternate expression.
Precedence
Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole
expression may be enclosed in parentheses to override these precedence rules and form a subexpression.
Back References and Subexpressions
The back-reference \n, where n is a single digit, matches the substring previously matched by the nth
parenthesized subexpression of the regular expression.
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead
use the backslashed versions \?, \+, \{, \|, \(, and \).
Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead,
so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.
GNU grep -E 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 command grep -E '{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.
Regular expressions are awesome!