Regular Expression
A regular expression is an object that can either be constructed with the RegEx constructor or written as a literal value by enclosing a pattern in a forward slash (/) characters. The syntaxes for creating a regular expression are shown below.
// using regular expression constructor
new RegExp(pattern[, flags]);
// using literals
/pattern/modifiers
The flags are optional while creating a regular expression using literals. Example of creating identical regular using above mentioned method.
let re1 = new RegExp("xyz");
let re2 = /xyz/;
Both ways will create a regex object and have the same methods and properties. There are cases where we might need dynamic values to create a regular expression, in that case, literals won't work and have to go with the constructor.
In cases where we want to have a forward slash to be a part of a regular expression, we have to escape the forward slash (/) with backslash (\).
The different modifiers that are used to perform case-insensitive searches are:
g- global search (finds all matches instead of stopping after the first match)i- case insensitive searchm- multiline matching
Brackets are used in a regular expression to find a range of characters. Some of them are mentioned below.
[abc]- find any character between the brackets[^abc]- find any character, not between the brackets[0-9]- find any digit between the bracket[^0-9]- find any character, not between the brackets (non-digit)(x|y)- find any of the alternatives separated by |
Metacharacters are special character that has special meaning in the regular expression. These characters are further described below:
| Metacharacter | Description |
|---|---|
. |
Match a single character excpet newline or a terminator |
\w |
Match a word character (alphanumeric character [a-zA-Z0–9_]) |
\W |
Match a non word character (same as [^a-zA-Z0–9_]) |
\d |
Match any digit character( same as [0-9]) |
\D |
Match any non digiti character |
\s |
Match a whitespace character (spaces, tabs etc) |
\S |
Match a non whitespace character |
\b |
Match at the begining / end of a word |
\B |
Match but not at the begining / end of a word |
\0 |
Match a NULL character |
| Match a new line character | |
\f |
Match a form feed character |
| Match a carriage return character | |
| Match a tab character | |
\v |
Match a tab vertical character |
\xxx |
Match a character specified by an octal number xxx |
\xdd |
Match a character specified by a hexadecimal number dd |
\udddd |
Match Unicode character specified by a hexadecimal number dddd |
Properties and methods supported by RegEx are listed below.
| Name | Description |
|---|---|
constructor |
Returns function that created RegExp object's protype |
global |
Checks if the g modifier is set |
ignoreCase |
Checksi if the i modifier is set |
lastIndex |
Specifies the index at which to start the next match |
multiline |
Checksi if the m modifier is set |
source |
Returns the text of the sttring |
exec() |
Test for the match and returns the first match, if no match then it returns null |
test() |
Test for the match and returns the true or false |
toString() |
Returns the string value of the regular exression |
A complie() method complies the regular expression and is deprecated.
Some examples of regular expressions are shown here.
let text = "The best things in life are free";
let result = /e/.exec(text); // looks for a match of e in a string
// result: e
let helloWorldText = "Hello world!";
// Look for "Hello"
let pattern1 = /Hello/g;
let result1 = pattern1.test(helloWorldText);
// result1: true
let pattern1String = pattern1.toString();
// pattern1String : '/Hello/g'