Regular Expressions 101

Explanation

Pro

Group d(?<d> [0-9] [-_ –.]* ){4}

{4}repeats the group contents below exactly 4 times:

Character Class[0-9]

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

Character Class[-_ –.]*

*matches any number of characters from the set below:

-the literal - (45₁₀ / 55₈ / 2D₁₆)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

the literal (32₁₀ / 40₈ / 20₁₆)

–the literal – (8211₁₀ / 20023₈ / 2013₁₆)

.the literal . (46₁₀ / 56₈ / 2E₁₆)

Comment: subpattern definition

Positive Lookahead(?= \g<d>{4} (?: \g<d> (?: \g<d>{3} )? )? # allowed digit sequences (?: \g<d>+ (*SKIP) (*F) )? # skip the substring if digits remain )

\g<d>{4}calls group d as a subroutine, repeated exactly 4 times

Non-Capturing Group(?: \g<d> (?: \g<d>{3} )? )?

?repeats the group contents below at most once:

\g<d>calls group d as a subroutine

Non-Capturing Group(?: \g<d>{3} )?

?repeats the group contents below at most once:

\g<d>{3}calls group d as a subroutine, repeated exactly 3 times

Comment: allowed digit sequences

Non-Capturing Group(?: \g<d>+ (*SKIP) (*F) )?

?repeats the group contents below at most once:

\g<d>+calls group d as a subroutine, repeated at least once

(*SKIP)acts like (*PRUNE), but the next unanchored match attempt starts where (*SKIP) was reached instead of at the next subject character

(*F)forces the current match attempt to fail at this position

Comment: skip the substring if digits remain

\g<d>*calls group d as a subroutine, repeated any number of times

Positive Lookahead(?= \g<d>{4} )

\g<d>{4}calls group d as a subroutine, repeated exactly 4 times

Comment: backtrack until there're 4 digits at the end

Global pattern flags

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

u modifier: unicode. Treats the pattern and subject as UTF-16

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

Match Information

Detailed match information will be displayed here automatically.

Quick Reference

25 tokens

Regular Expression

(?<d> [0-9] [-_ –.]* ){4} # subpattern definition

(?= 
    \g<d>{4} (?: \g<d> (?: \g<d>{3} )? )? # allowed digit sequences
    (?: \g<d>+ (*SKIP) (*F) )? # skip the substring if digits remain
)

\g<d>* (?= \g<d>{4} ) # backtrack until there're 4 digits at the end

New

valid:
12345678
123456789
123456789012
123-456789
123 45 6789
123 – 45 – 6789

invalid:
1234567
1234567890
12345678901
1234567890123