Regular Expressions 101

Explanation

Pro

^start of line

Negative Lookahead(?!.*?_.*?)

.*?any character (except for line terminators), repeated any number of times

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

.*?any character (except for line terminators), repeated any number of times

Negative Lookahead(?!(?:[\w]+?\.)?\-[\w\.\-]*?)

Non-Capturing Group(?:[\w]+?\.)?

?repeats the group contents below at most once:

Character Class[\w]+?

+?matches at least one character from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Character Class[\w\.\-]*?

*?matches any number of characters from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Negative Lookahead(?![\w]+?\-\.(?:[\w\.\-]+?))

Character Class[\w]+?

+?matches at least one character from the set below:

\wany word character

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Non-Capturing Group(?:[\w\.\-]+?)

Character Class[\w\.\-]+?

+?matches at least one character from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Positive Lookahead(?=[\w])

Character Class[\w]

\wany word character

Positive Lookahead(?=[\w\.\-]*?\.+[\w\.\-]*?)

Character Class[\w\.\-]*?

*?matches any number of characters from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.+the literal . (46₁₀ / 56₈ / 2E₁₆) , repeated at least once

Character Class[\w\.\-]*?

*?matches any number of characters from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Negative Lookahead(?![\w\.\-]{254})

Character Class[\w\.\-]{254}

{254}matches exactly 254 characters from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Negative Lookahead(?!(?:\.?[\w\-\.]*?[\w\-]{64,}\.)+?)

Non-Capturing Group(?:\.?[\w\-\.]*?[\w\-]{64,}\.)+?

+?repeats the group contents below at least once:

\.?optionally matches the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[\w\-\.]*?

*?matches any number of characters from the set below:

\wany word character

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[\w\-]{64,}

{64,}matches at least 64 characters from the set below:

\wany word character

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[\w\.\-]+?

+?matches at least one character from the set below:

\wany word character

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

Negative Lookbehind(?<![\w\-\.]*?\.[\d]+?)

Character Class[\w\-\.]*?

*?matches any number of characters from the set below:

\wany word character

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[\d]+?

+?matches at least one character from the set below:

\da digit

Positive Lookbehind(?<=[\w\-]{2,})

Character Class[\w\-]{2,}

Negative Lookbehind(?<![\w\-]{25})

$end of line

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Match Information

Detailed match information will be displayed here automatically.

Quick Reference

26 tokens

Regular Expression

^(?!.*?_.*?)(?!(?:[\w]+?\.)?\-[\w\.\-]*?)(?![\w]+?\-\.(?:[\w\.\-]+?))(?=[\w])(?=[\w\.\-]*?\.+[\w\.\-]*?)(?![\w\.\-]{254})(?!(?:\.?[\w\-\.]*?[\w\-]{64,}\.)+?)[\w\.\-]+?(?<![\w\-\.]*?\.[\d]+?)(?<=[\w\-]{2,})(?<![\w\-]{25})$

# TEST

## List of valid domains:
www.google.com
google.com
mkyong123.com
mkyong-info.com
sub.mkyong.com
sub.mkyong-info.com
mkyong.com.au
g.co
a.12E
mkyong.t.t.co
xn--stackoverflow.com
stackoverflow.xn--com
xn--d1ai6ai.xn--p1ai
stackoverflow.co.uk
a.xn--wgbh1c
1.2.3.4.com
x.XN--VERMGENSBERATUNG-PWB
abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com
www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com
abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcde.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com

## List of invalid domains:
http://google.com
http://www.google.com
dot.
space com
under_score.com
underscore.c_om
-dash.com
dash-.com
sub.-dash.com
sub-.dash.com
-.com
-com
.com
com
mx.gmail.com.
mkyong.t.t.c
mkyong,com
mkyong.com/users
a.123
x.XN--VERMGENSBERATUNG-PWBB
abcdefghijklmnopqrstuvwxyzabcdef

Substitution

# TEST

## List of valid domains:
www.google.com
google.com
mkyong123.com
mkyong-info.com
sub.mkyong.com
sub.mkyong-info.com
mkyong.com.au
g.co
a.12E
mkyong.t.t.co
xn--stackoverflow.com
stackoverflow.xn--com
xn--d1ai6ai.xn--p1ai
stackoverflow.co.uk
a.xn--wgbh1c
1.2.3.4.com
x.XN--VERMGENSBERATUNG-PWB
abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com
www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com
abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcde.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com

## List of invalid domains:
http://google.com
http://www.google.com
dot.
space com
under_score.com
underscore.c_om
-dash.com
dash-.com
sub.-dash.com
sub-.dash.com
-.com
-com
.com
com
mx.gmail.com.
mkyong.t.t.c
mkyong,com
mkyong.com/users
a.123
x.XN--VERMGENSBERATUNG-PWBB
abcdefghijklmnopqrstuvwxyzabcdef