Very basics of regexp for mod_rewrite

Regex vocabulary
The following are the minimal building blocks you will need, in order to write regular expressions and RewriteRules. They certainly do not represent a complete regular expression vocabulary, but they are a good place to start, and should help you read basic regular expressions, as well as write your own.

Character	Meaning	Example
.	Matches any single character	c.t will match cat, cot, cut, etc.
+	Repeats the previous match one or more times	a+ matches a, aa, aaa, etc
*	Repeats the previous match zero or more times.	a* matches all the same things a+ matches, but will also match an empty string.
?	Makes the match optional.	colou?r will match color and colour.
^	Called an anchor, matches the beginning of the string	^a matches a string that begins with a
$	The other anchor, this matches the end of the string.	a$ matches a string that ends with a.
( )	Groups several characters into a single unit, and captures a match for use in a backreference.	(ab)+ matches ababab - that is, the +applies to the group. For more on backreferences see below.
[ ]	A character class - matches one of the characters	c[uoa]t matches cut, cot or cat.
[^ ]	Negative character class - matches any character not specified	c[^/]t matches cat or c=t but not c/t

In mod_rewrite the ! character can be used before a regular expression to negate it. This is, a string will be considered to have matched only if it does not match the rest of the expression.

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Here's the very basics of regexp (expanded from the Apache mod_rewrite documentation)..

Escaping:
\char escape that particular char
For instance to specify special characters.. [].()\ etc.

Text:
. Any single character (on its own = the entire URI)
[chars] Character class: One of following chars
[^chars] Character class: None of following chars
text1|text2 Alternative: text1 or text2 (i.e. "or")
e.g. [^/] matches any character except /
(foo|bar)\.html matches foo.html and bar.html

Quantifiers:
? 0 or 1 of the preceding text
* 0 or N of the preceding text (hungry)
+ 1 or N of the preceding text
e.g. (.+)\.html? matches foo.htm and foo.html
(foo)?bar\.html matches bar.html and foobar.html

Grouping:
(text) Grouping of text
Either to set the borders of an alternative or for making backreferences where the nthe group can
be used on the target of a RewriteRule with $n
e.g. ^(.*)\.html foo.php?bar=$1

Anchors:
^ Start of line anchor
$ End of line anchor
An anchor explicitly states that the character right next to it MUST
be either the very first character ("^"), or the very last character ("$") of the URI string to match against the pattern, e.g.. ^foo(.*) matches foo and foobar but not eggfoo
(.*)l$ matches fool and cool, but not foo