Regular Expression

GREP = “Global Regular Expression Print”

Online Regex Editor: regexpal( http://www.regexpal.com/ )

Modes
Standard : /re/
Global : /re/g
Case Insensitive : /re/i
Multiline : /re/m
Dot-matches-all : /re/s

Literals 
a matches a 
b matches b
like this so on…

Meta-characters 
\ . * + — {} [] ^$ | ? () : ! =

Wildcard Meta-Character
/h . t/ matches “hat”, “hot” but not “heat”
/9 . 00/ matches “9.00” but also “9500” or “9–00”

Escaping Meta-Characters 
/9 \. 00/ matches “9.00” but not “9500” or “9–00”

Other Special Characters
Spaces
Tab
Line Return (\r,\n,\r\n)
ASCII or ANSI Codes

Character Sets
/gr[ea]y/ matches “gray” & “grey”
/gr[ea]t/ does not matches with “great”

Character Ranges
[0–9]
[A-Za-z]

Negative Characters
^ Negate a character set
[^aeiou] — consonants
see[^mn] matches “seek” and “sees” but not “seem” and “seen”

Meta-characters inside character sets
/h[abc.xyz]t/ matches with “hat” & “h.t” but not to “hot”

Shorthand Characters
\d digit [0–9]
\w word character [0–9A-Za-z_]
\s whitespace [ \t\r\n]
\D not digit [^0–9]
\W not word character [^0–9A-Za-z_]
\S not whitespace [^ \t\r\n]

POSIX Bracket Expression

Repetition Meta Characters 
* preceding items zero or more times
+ preceding items one or more times
? preceding items zero or one times
/cats*/ matches “cat”, “cats” , “catsssss”
/cats+/ matches “cats” and “catssss” but not “cat”
/cats?/ matches “cat” and “cats” but not to “catssss”

Quantified Repetition
{min,max}
/d{4,9} matches with word length from 4 to 9
/d{4} matches with word exactly with length 4
/d{4,} marches with length from 4 to infinity

Grouping Meta Characters
/(abc)+/ matches “abc” and “abcabcabc”
/(in)?dependent / matches “dependent” and “independent”

Alternation Meta Character
| match previous or next expression
It is an “OR” operator

Start and End Anchors
^ Start of string line
$ End of string line

Word Boundaries 
/b word boundary (start/end )
/B not a word boundary

Back references
\1 through \9

Positive Lookahead Assertions
?=
if lookahead assertion fails that match also fails.
Zero width does not include in any group
Syntax : /(?=regex)/

Negative Lookahead Assertions
Syntax : /(?!regex)/

Look Behind Assertion
/(?<=regex)/
/(?<!regex)/