Regular Expressions 101

Explanation

Pro

Definition Group(?(DEFINE) (?<let_dig> [a-z0-9]) # alpha digit (?<let_dig_hyp> [a-z0-9-]) # alpha digit and hyphen (?<ldh_str> (?&let_dig_hyp){0,61}(?&let_dig)) # domain label end (?<label> (?&let_dig)((?&ldh_str))?) # domain label (?<domain> (?&label)(\.(?&label)){0,126}\.?) # domain name )

Group let_dig(?<let_dig> [a-z0-9])

Character Class[a-z0-9]

a-zone character from a (97₁₀) to z (122₁₀) (case insensitive)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Comment: alpha digit

Group let_dig_hyp(?<let_dig_hyp> [a-z0-9-])

Character Class[a-z0-9-]

a-zone character from a (97₁₀) to z (122₁₀) (case insensitive)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

-the literal - (45₁₀ / 55₈ / 2D₁₆)

Comment: alpha digit and hyphen

Group ldh_str(?<ldh_str> (?&let_dig_hyp){0,61}(?&let_dig))

(?&let_dig_hyp){0,61}calls group let_dig_hyp as a subroutine, repeated between 0 and 61 times

(?&let_dig)calls group let_dig as a subroutine

Comment: domain label end

Group label(?<label> (?&let_dig)((?&ldh_str))?)

(?&let_dig)calls group let_dig as a subroutine

Group 5((?&ldh_str))?

?repeats the group contents below at most once:

(?&ldh_str)calls group ldh_str as a subroutine

Comment: domain label

Group domain(?<domain> (?&label)(\.(?&label)){0,126}\.?)

(?&label)calls group label as a subroutine

Group 7(\.(?&label)){0,126}

{0,126}repeats the group contents below between 0 and 126 times:

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

(?&label)calls group label as a subroutine

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

Comment: domain name

^start of string

(?&domain)calls group domain as a subroutine

$end of string, or before its final line terminator

Global pattern flags

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

Match Information

Detailed match information will be displayed here automatically.

Quick Reference

25 tokens

Regular Expression

(?(DEFINE)
        (?<let_dig> [a-z0-9])                         # alpha digit
        (?<let_dig_hyp> [a-z0-9-])                    # alpha digit and hyphen
        (?<ldh_str> (?&let_dig_hyp){0,61}(?&let_dig)) # domain label end
        (?<label> (?&let_dig)((?&ldh_str))?)          # domain label
        (?<domain> (?&label)(\.(?&label)){0,126}\.?)  # domain name
    )
        ^(?&domain)$

www.ulb.ac.be

Extraction

www.ulb.ac.be