Regular expression

Syntax

Brackets

Patterns that find a range of characters in the string

• […] Any of the characters encapsulated.
• [^…] Not any character encapsulated.
• [0-9]
• [a-z]

Quantifiers

Special characters or flags that specify the position or frequency of a certain pattern

Frequency

• p+ one or more times
• p* zero or more times
• p? maximum once
• p{n} in a sequence n times
• p{m,n} minimum m times and max n times

Positional

• ^p p should exist at the start of the string
• p$ p should exist at the end of the string.

Lookaround

Assertions that see if a character match is possible or not, and accordingly return boolean values without consuming any part of the string.

* $p(?=q)$ Assert that p should be immediately be followed by q
* $p(?!q)$ Assert that p should NOT be immediately followed by q
* $p(?<=q)$ Assert that p should immediately be preceded by q
* $p(?<!q)$ Assert that p should NOT immediately be preceded by q

Character literals

representations for characters and special characters.

• Alphanumeric Any alphabet or numeric character itself e.g. a is any character which is the letter a
• \0 A null character
• \t A tab character
• \n A newline character
• \v A vertical tab character
• \f A form feed character (next page character)
• \r A carriage return character

Metacharacters

character representations for a group of characters falling under the same categories.

• . Any single character ([^\n])
• \s A white space character like tab and spaces ([ \n\r\t\f])
• \S Any character which is not a white space character ([^ \n\r\t\f])
• \d Any numeric character ([0-9])
• \D Any non-numeric character ([^0-9])
• \w Any word character ([a-zA-Z0-9_])
• \W Any non-word character ([^a-zA-Z0-9_])

RegExp object

exec method

It runs the regular expression on a string passed as an argument and returns the found text as an object

test method

It runs the regular expression on a string passed as an argument and returns a boolean value telling if a match was found or not.

String methods with RE

Search method

It takes a query-string as an argument and searches within the string to return the character index at which the query-string starts. It returns “-1” if no match is found.

replace method

It takes a query-string as an argument and searches within the string to replace the matching substring.

RE example

// Check if the password is strong password.
// Length: .{6,} (Minimum 6 characters)
// Uppercase letter: .*[A-Z] (A-Z)
// Lowercase letter: .*[a-z] (a-z)
// Numeric digit: .*\d (0-9)
// Special Character: .*[\W] (any character other than {0-9, a-z, A-Z})
var isSafe = function (password) {
  var regex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[\W]).{6,}$/;
  return regex.test(password); // return regex.test(password) after completing regex
};