Regular Expression in JS

In javascript, regex use the delimiter /.

The syntax is like this: /<pattern>/<modifier>

Modifiers

g - Perform a global match (find all matches rather than stopping after the first match)
i - Perform case-insensitive matching
m - Perform multiline matching

Pattern - Anchors

^ - means pattern in the beginning of words
$ - means pattern in the last of words

Pattern - Brackets

In brackets, ^ means NOT, instead of the beginning of the line.

[abc] - Find any character between the brackets
[^abc] - Find any character NOT between the brackets
[0-9] - Find any character between the brackets (any digit)
[^0-9] - Find any character NOT between the brackets (any non-digit)
(x|y) - Find any of the alternatives specified; () defines a group

Pattern - Metacharacters

Metacharacters are characters with a special meaning

. - Find a single character, except newline or line terminator
\w - Find a word character, including a-z, A-Z, 0-9, and _
\W - Find a non-word character
\d - Find a digit
\D - Find a non-digit character
\s - Find a whitespace character, including a space / tab / new line character
\S - Find a non-whitespace character
\b - Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b
\B - Find a match, but not at the beginning/end of a word
\n - Find a new line character
\t - Find a tab character

Pattern - Quantifiers

n+ - Matches any string that contains at least one n
n* - Matches any string that contains zero or more occurrences of n
n? - Matches any string that contains zero or one occurrences of n
n{X} - Matches any string that contains a sequence of X n’s
n{X,Y} - Matches any string that contains a sequence of X to Y n’s
n{X,} - Matches any string that contains a sequence of at least X n’s
?=n - Matches any string that is followed by a specific string n
?!n - Matches any string that is not followed by a specific string n

Some Common Patterns

Match a positive integer

^\d+$
- postive integer starting will 0 is excluded (e.g. 012)
^[1-9]\d*$
^[1-9][0-9]*$

Match a negative integer

^-[1-9]\d*$
^-[1-9][0-9]*$

Match an integer

^-?[1-9]\d*$
- 0 is excluded
^-?[1-9][0-9]*$
- 0 is excluded
^(0|-?[1-9][0-9]*)$
^(0|-?[1-9]\d*)$

Match a decimal number, e.g., 3.5

^-?(0|[1-9]\d*)\.\d+$

Match a integer or decimal number

^[0-9]\d*(\.\d+)?$
- positive numbers only
^-?[0-9]\d*(\.\d+)?$
- cover both postives or negatives

Match an email

([\w\.-]+)@([\w\.-]+)(\.[\w\.]+)

Match a Date (dd mm yyyy, d/m/yyyy, etc.):

1	^([1-9]\|0[1-9]\|[12][0-9]\|3[01])\D([1-9]\|0[1-9]\|1[012])\D(19[0-9][0-9]\|20[0-9][0-9])$

Match Year 1900 - 2099

1	^(19\|20)[\d]{2,2}$

More Examples:

https://digitalfortress.tech/tricks/top-15-commonly-used-regex/

Using Regex in JS

Search and Replace

search() uses a regular expression to search for a match and returns the position of the match.
replace() returns a modified string where the pattern is replaced.

Example:

var str = "Vines Note :)";
var n = str.search("Note");
console.log(n); // 6

var res = str.replace(/note/i, "Book")
console.log(res); // Vines Book :)

Test and Exec

test() searches a string for a pattern, and returns boolean value whether it exists or not.
exec() searches a string for a specified pattern and returns the found text as an object.

Example:

var pattern = /e/g; 
var str = "The best things in life are free!";
console.log(pattern.test(str));
console.log(/e/g.test(str));

/*
true
true
*/

var str = "is this his shirt"; 
var pat = /is/g; 
var result; 
while ((result = pat.exec(str)) != null) { 	
  console.log(result+" "+ pat.lastIndex); 
}

/*
is 2
is 7
is 11
*/

Regular Expression in PHP

In PHP, regex also use the same delimiter /.

The syntax is like this: /<pattern>/<modifier>

For the regex rules, same as JS.

Using Regex in PHP

PHP provides a variety of functions that allow you to use regular expressions.

preg_match($pattern, $str) - tell you whether a string contains matches of a pattern (1 or 0)
preg_match($pattern, $str) - tell you how many matches of a pattern
preg_replace($pattern, "<replacement>", $str) - replace all of the matches of the pattern with another string
preg_split($pattern, $str) - breaks a string into an array using matches of a regular expression as separators.

Examples:

<?php
    $str = "The rain in SPAIN falls mainly on the plains.";
    $pattern = "/ain/i";

    echo preg_match($pattern, $str);

    // Outputs 1

    echo preg_match_all($pattern, $str);

    // Outputs 4

    echo preg_replace($pattern, "??", $str);

    // Outputs "The r? in SP? falls m?ly on the pl?s."
?>

<?php
	$date = "2020-10-15 16:30:00";
	$pattern = "/[-\s:]/";
	$components = preg_split($pattern, $date);
	for($i=0; $i<count($components); $i++)
    	echo "$components[$i] ,";
	//Output "2020 10 15 16 30 00"
?>