Regular Expression

Structure of RE :
Anchors are used to specify the position of the pattern in relation to a line of text.
Character Sets match one or more characters in a single position.
Modifiers specify how many times the previous character set is repeated.

There are also two types of regular expressions:
- "Basic" regular expression,
- "extended" regular expression.

Utility	Regular Expression Type
vi	Basic
sed	Basic
grep	Basic
csplit	Basic
dbx	Basic
dbxtool	Basic
more	Basic
ed	Basic
expr	Basic
lex	Basic
pg	Basic
nl	Basic
rdist	Basic
awk	Extended
nawk	Extended
egrep	Extended
EMACS	EMACS Regular Expressions
PERL	PERL Regular Expressions

Anchor Characters: ^ and $

Pattern	Matches
^A	"A" at the beginning of a line
A$	"A" at the end of a line
A^	"A^" anywhere on a line
$A	"$A" anywhere on a line
^^	"^" at the beginning of a line
$$	"$" at the end of a line

Match any character with .
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
^.$

Regular
Expression Meaning
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. - A single character (except newline)
^ - Beginning of line
$ - End of line
[...] - Range of characters
* - zero or more duplicates
\< - Beginning of word
\> - End of word
\(..\) - Remembers pattern
\1..\9 - Recalls pattern
_+ - One or more duplicates
? - Zero or one duplicate
\{M,N\} - M to N Duplicates
(...|...) Shows alteration
\(...\|...\) Shows alteration
\w - Matches a letter in a word
\W - Opposite of \w

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