Hand out - Javascript - Regular Expression

Introduction:
This post is to know regular expression, see usage in sample examples using JavaScript. Plan is to update this post every time a regular expression is solved. Considering this as a quick hand out to refresh memories as well as a list regular expression which are encountered at some point of development life time.


What is regular expression:
A regular expression is a sequence of characters that forms a search pattern to match a combination in strings. In JavaScript regular expression is an object which can be created explicitly or implicitly.

Regular expression creation: 

Option-01: implicit way -

var regExp = /[0-9]/;

Option-02: explicit way- 

var regExp = new RegExp('[0-9]');

Note: better to use option-01 as option-02 has implication on execution speed.


Various patterns:

1. Direct match - /direct/. This pattern will find a match in string 'Looking for a direct match' but not in 'Misspelled the word diract'.

2. Special characters - 

Pattern	Meaning/example	Matching String	Non matching string
\	Escape character
^	matches beginning of input. /^A/	Apple	Banana
$	matches end of input. /a$/	Banana	apple
*	matches preceding expression 0 or many times. /p*/	- Apple - Orange
+	matches preceding expression 1 or many times. /p+/	- Apple - Peach	- Orange, any string not have p in it
?	matches preceding expression 0 or 1 times. /p?/	- Peach - Pear	- Apple -
()	matches and remembers it. Also called capturing parentheses. /(p)/		Table Cell
(?:)	Matches 'x' but does not remember the match
(?=)	Look ahead. /A(?=p)/. Match only if A (capital) followed by p.	- Apple - Apricot	- Avocado - apple
(?!)	negated look ahead. /A(?!p)/. Match only if A (capital) doesn't follow by p.	- Avocado - apple	- Apple - Apricot
|	matches either one. /A|a/.	- Apple - apple	- one - Two - Three
{}	matches exactly that many occurrences. /p{2}/. Has to be positive number.	- Apple - Pepper	- Apricot - Pear - Peach
{ , }	matches at least first and at most second. /p{1,2}/.	- Apple - Pepper	- Apricot - Pear - Peach
[]	Character set. /[Pe|pe]/.	- Pepper - Pear - Peach	- Apricot - Apple
[^ ]	Negated character set. /[^pe]/.	- Pear - Peach - Apple - Apricot	- Pepper
\b	matches word boundary where word character not preceded or followed wit any character. /\bp/.	- pear - peach	-Apple
\d	matches digit character. /\d/. equals to /[0-9]/.	- 2apples - 3 Pears	- Apple - Pear
\D	match non digit character. /\D/. equals to /[^0-9]/.	- Apple - Pear	- 2apples - 3 Pears
\s	matches white space. /\s/.	- 2 Apples - Apple or orange	- AppleOrOrange - PearAndPeach
\S	matches a single character other than white space. /\S/.
\w	matches any alphanumeric words including _. /\w/. equals to /[A-Za-z0-9_]/.	- &^%$A match A
\W	Matches any non-word character. /\W/. equals to /[^A-Za-z0-9_]/.	- Appl# matches #
Note: the list is not complete. Check Resource links for complete list

Regular Expression properties:

Property Name	Description
lastIndex	The index at which to start the next match
ignoreCase	Indicates if the "i" flag was used to ignore case. Returns true or false
global	Indicates if the "g" flag was used for global match. Returns true or false
multiline	Indicates if the "m" flag was used match from multiple lines. Returns true or false
source	Returns the pattern used in regular expression
split	This method is in String. If match found, returns array of substrings.

Methods related to regular expression:

Method Name	Description
exec	that executes a search for a match in a string. It returns an array
re.exec(str) - matches the regular expression (re) against the input parameter string (str). Properties: [0] - The full string of characters matched [1], ...[n ] - matched substrings index - zero based, index of matched string input - the string match happening against
test	tests for a match in a string and returns true or false
re.test(str) - regular expression (re), input parameter string (str).
match	This method is in String. Executes a search and returns an array of information or null on a mismatch
str.match(re) - regular expression (re) pattern to match on string (str).
search	This method is in String. Returns the index if match found, else -1.
str.search(re) - regular expression (re) pattern to search from string (str).
replace	This method is in String. Replaces matched substring with replacement substring.
str.replace(re, newSubStr) - replace substring from string (str) if match found based on regular expression (re) pattern with new sub string (newSubStr). Note: a function can be used instead of newSubStr as replacement
split	This method is in String. If match found, returns array of substrings.
str.split([separator[, limit]]) - Splits the string (str) based on separator. Limit value can be specified which is optional. If used, it splits on every match on separator but truncates the array to meet the limit.

Regular expression with flags:

Syntax: /regular expression/flag;
Flags:

i - case ignore search

g - global search

m - multi line search


Examples:

1. Replace all instances of e or E in a String - /e|E/g.

2. 




Resources:
1. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
2.