This article was originally published at: blog.bitsrc.io/a-beginners-guide-to-regular..

When you first encounter Regular Expressions, they may seem like a random string of gibberish. While they might look awkward (with somewhat confusing syntax), they are also extremely useful.

The truth is properly understanding regular expressions will make you a much more effective programmer. To fully understand the regex world, you first need to learn the basics concepts, on which you can later build.

So, without further ado, let's get started :)

What are Regular Expressions?

Regular expressions are a way to describe patterns in a string data. They form a small language of its own, which is a part of many programming languages like JavaScript, Perl, Python, PHP, and Java.

Regular expressions allow you to check a string of characters like an e-mail address or password for patterns, to see so if they match the pattern defined by that regular expression and produce actionable information.

Creating a Regular Expression

There are two ways to create a regular expression in JavaScript. It can be either created with the RegExp constructor or by using forward slashes ( / ) to enclose the pattern.

Regular Expression Constructor:

Syntax: new RegExp(pattern[, flags])

Example:

var regexConst = new RegExp('abc');

Regular Expression Literal:

Syntax: /pattern/flags

Example:

var regexLiteral = /abc/;

• Here the flags are optional, I will explain these later in this article.

There might also be cases where you want to create regular expressions dynamically, in which case regex literal won’t work, so you have to use a regular expression constructor.

No matter which method you choose, the result is going to be a regex object. Both regex objects will have the same methods and properties attached to them.

Since forward slashes are used to enclose patterns in the above example, you have to escape the forward slash **( / )** with a backslash **( \ )** if you want to use it as a part of the regex.

Regular Expressions Methods

There are mainly two methods for testing regular expressions.

RegExp.prototype.test()

This method is used to test whether a match has been found or not. It accepts a string which we have to test against a regular expression and returns true or false depending upon if the match is found or not.

For example:

var regex = /hello/;

var str = 'hello world';

var result = regex.test(str);

console.log(result);

// returns true

RegExp.prototype.exec()

This method returns an array containing all the matched groups. It accepts a string that we have to test against a regular expression.

For example:

var regex = /hello/;

var str = 'hello world';

var result = regex.exec(str);

console.log(result);

// returns \[ 'hello', index: 0, input: 'hello world', groups: undefined \]

// 'hello' -> is the matched pattern.  
// index: -> Is where the regular expression starts.  
// input: -> Is the actual string passed.
`

We are going to use the test() method in this article.

Simple Regex Patterns

It is the most basic pattern, which simply matches the literal text with the test string. For example:

var regex = /hello/;

console.log(regex.test('hello world'));

// true

Special Characters

Up until now, we’ve created simple regular expression patterns. Now, let’s tap into the full power of regular expressions when handling more complex cases.

For example, instead of matching a specific email address let’s say we’d like to match many email addresses. That’s where special characters come into play. There are special symbols and characters that you have to memorize to fully understand the regular expressions.

Flags:

Regular expressions have five optional flags or modifiers. Let’s discuss the two most important flags:

• g — Global search, don’t return after the first match
• i — Case-insensitive search

You can also combine the flags in a single regular expression. Note that their order doesn’t have any effect on the result.

Let’s look at some code examples:

Regular Expression Literal — Syntax /pattern/flags

var regexGlobal = /abc/g;

console.log(regexGlobal.test('abc abc'));

// it will match all the occurrences of 'abc', so it won't return   
// after the first match.

var regexInsensitive = /abc/i;

console.log(regexInsensitive.test('Abc'));

// returns true because the case of string characters doesn't matter   
// in case-insensitive search.

Regular Expression Constructor — Syntax new RegExp('pattern', 'flags')

var regexGlobal = new RegExp('abc','g')

console.log(regexGlobal.test('abc abc'));

// it will match all the occurrences of 'abc', so it won't return // after the first match.

var regexInsensitive = new RegExp('abc','i')

console.log(regexInsensitive.test('Abc'));

// returns true, because the case of string characters doesn't matter in a case-insensitive search.

Character groups:

Character set [xyz] — A character set is a way to match different characters in a single position, it matches any single character in the string from characters present inside the brackets. For example:

var regex = /\[bt\]ear/;

console.log(regex.test('tear'));  
// returns true  
console.log(regex.test('bear'));  
// return true  
console.log(regex.test('fear'));  
// return false

Note — All the special characters except for caret (^) (Which has an entirely different meaning inside the character set) lose their special meaning inside the character set.

Negated character set [^xyz] — It matches anything that is not enclosed in the brackets. For example:

var regex = /\[^bt\]ear/;

console.log(regex.test('tear'));  
// returns false  
console.log(regex.test('bear'));  
// return false  
console.log(regex.test('fear'));  
// return true

Ranges [a-z] — Suppose we want to match all of the letters of an alphabet in a single position, we could write all the letters inside the brackets, but there is an easier way and that is ranges. For example: [a-h] will match all the letters from a to h. Ranges can also be digits like [0-9] or capital letters like [A-Z].

var regex = /\[a-z\]ear/;

console.log(regex.test('fear'));  
// returns true

console.log(regex.test('tear'));  
// returns true

Meta-characters — Meta-characters are characters with a special meaning. There are many meta characters but I am going to cover the most important ones here.

• \d — Match any digit character ( same as [0-9] ).
• \w — Match any word character. A word character is any letter, digit, and underscore. (Same as [a-zA-Z0–9_] ) i.e alphanumeric character.
• \s — Match a whitespace character (spaces, tabs etc.).
• \t — Match a tab character only.
• \b — Find a match at the beginning or end of a word. Also known as the word boundary.
• . — (period) Matches any character except for the new line.
• \D — Match any non-digit character (same as [^0–9]).
• \W — Match any non-word character (Same as [^a-zA-Z0–9_] ).
• \S — Match a non-whitespace character.

Quantifiers: — Quantifiers are symbols that have a special meaning in a regular expression.

• + — Matches the preceding expression 1 or more times.

var regex = /\\d+/;  
console.log(regex.test('8'));  
// true

console.log(regex.test('88899'));  
// true

console.log(regex.test('8888845'));  
// true

• * —Matches the preceding expression 0 or more times.

var regex = /go\*d/;

console.log(regex.test('gd'));  
// true

console.log(regex.test('god'));  
// true

console.log(regex.test('good'));  
// true

console.log(regex.test('goood'));  
// true

• ? — Matches the preceding expression 0 or 1 time, that is preceding pattern is optional.

var regex = /goo?d/;

console.log(regex.test('god'));  
// true

console.log(regex.test('good'));  
// true

console.log(regex.test('goood'));  
// false

• ^ — Matches the beginning of the string, the regular expression that follows it should be at the start of the test string. i.e. the caret (^) matches the start of the string.

var regex = /^g/;

console.log(regex.test('good'));  
// true

console.log(regex.test('bad'));  
// false

console.log(regex.test('tag'));  
// false
`

• $ — Matches the end of the string, that is the regular expression that precedes it should be at the end of the test string. The dollar ($) sign matches the end of the string.

var regex = /.com$/;

console.log(regex.test('test@testmail.com'));  
// true

console.log(regex.test('test@testmail'));  
// false

• {N} — Matches exactly N occurrences of the preceding regular expression.

var regex = /go{2}d/;

console.log(regex.test('good'));  
// true

console.log(regex.test('god'));  
// false

• {N,} — Matches at least N occurrences of the preceding regular expression.

var regex = /go{2,}d/;

console.log(regex.test('good'));  
// true

console.log(regex.test('goood'));  
// true

console.log(regex.test('gooood'));  
// true

• {N,M} — Matches at least N occurrences and at most M occurrences of the preceding regular expression (where M > N).

var regex = /go{1,2}d/;

console.log(regex.test('god'));  
// true

console.log(regex.test('good'));  
// true

console.log(regex.test('goood'));  
// false

Alternation X|Y — Matches either X or Y. For example:

var regex = /(green|red) apple/;

console.log(regex.test('green apple'));  
// true  
console.log(regex.test('red apple'));  
// true  
console.log(regex.test('blue apple'));  
// false

Note — If you want to use any special character as a part of the expression, say for example you want to match literal + or ., then you have to escape them with a backslash ( \ ). For example:

var regex = /a+b/;  // This won't work

var regex = /a\\+b/; // This will work

console.log(regex.test('a+b')); // true

Advanced

(x) — Matches x and remembers the match. These are called capturing groups. This is also used to create subexpressions within a regular expression. For example:

var regex = /(foo)bar\\1/;  
console.log(regex.test('foobarfoo'));  
// true

console.log(regex.test('foobar'));  
// false

\1 remembers and uses that match from the first subexpression within parentheses.

(?:x) — Matches x and does not remember the match. These are called non-capturing groups. Here \1 won’t work, it will match the literal \1.

var regex = /(?:foo)bar\\1/;  
console.log(regex.test('foobarfoo'));  
// false

console.log(regex.test('foobar'));  
// false

console.log(regex.test('foobar\\1'));  
// true

x(?=y) — Matches x only if x is followed by y. Also called a positive look ahead. For example:

var regex = /Red(?=Apple)/;

console.log(regex.test('RedApple'));  
// true

In the above example, a match will occur only if Red is followed by Apple.

Practicing Regex:

Let’s practice some of the concepts that we have learned above.

• Match any 10-digit number :

var regex = /^\\d{10}$/;

console.log(regex.test('9995484545'));  
// true

Let’s break that down and see what’s going on up there.

1. If we want to enforce that the match must span the whole string, we can add the quantifiers ^ and $. The caret ^ matches the start of the input string, whereas the dollar sign $ matches the end. So, it would not match if the string contained more than 10 digits.
2. \d matches any digit character.
3. {10} matches the previous expression, in this case, \d exactly 10 times. So, if the test string contains less than or more than 10 digits, the result will be false.

• Match a date with the following format DD-MM-YYYY or DD-MM-YY

var regex = /^(\\d{1,2}-){2}\\d{2}(\\d{2})?$/;  
console.log(regex.test('01-01-1990'));  
// true  
console.log(regex.test('01-01-90'));  
// true  
console.log(regex.test('01-01-190'));  
// false

Let’s break that down and see what’s going on up there.

1. Again, we have wrapped the entire regular expression inside ^ and $, so that the match spans the entire string.
2. ( start of the first subexpression.
3. \d{1,2} matches at least 1 digit and at most 2 digits.
4. - matches the literal hyphen character.
5. ) end of the first subexpression.
6. {2} match the first subexpression exactly two times.
7. \d{2} matches exactly two digits.
8. (\d{2})? matches exactly two digits. But it’s optional, so either year contains 2 digits or 4 digits.

• Matching Anything But a Newline

The expression should match any string with a format like abc.def.ghi.jkl where each variable a, b, c, d, e, f, g, h, i, j, k, l can be any character except new line.

var regex = /^(.{3}\\.){3}.{3}$/;

console.log(regex.test('123.456.abc.def'));  
// true

console.log(regex.test('1243.446.abc.def'));  
// false

console.log(regex.test('abc.def.ghi.jkl'));  
// true

Let’s break that down and see what’s going on up there.

1. We have wrapped the entire regular expression inside ^ and $ so that the match spans the entire string.
2. ( start of the first subexpression
3. .{3} matches any character except the new line for exactly 3 times.
4. \. matches the literal . period
5. ) end of the first subexpression
6. {3} matches the first subexpression exactly 3 times.
7. .{3} matches any character except the new line for exactly 3 times.

Conclusion

Regular expression can be fairly complex at times, but having a proper understanding of the above concepts will help you understand more complex regex patterns easily. You can learn more about regex here and practice it here.

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