Use regex to match string in given condition

Question

[Edit] Note:

Shortest regex is the main question, not about back-reference.

Requirement:

Use shortest regex to match all string in following format:

<two digits><connect char><three digits><connect char><four digits>

For easy reading:

<two digits>
<connect char>
<three digits>
<connect char>
<four digits>

Conditions:

Match whole string, given that input string is single line.
Connect char may both omit, or are any of [-./ ] (not include []).
Two connect char must be the same in each matched string.
Shortest is important, performance is not important.

Example

Some valid string:

55.635.8828
72/683/1582
86 942 7682
581827998      // Both connect chars is omit

Some invalid string:

56.855/9856     // Two connect chars are different.
56 4559428      // Same as above

This short regex will match all valid string:

^\d{2}[-./ ]?\d{3}[-./ ]?\d{4}$

But it also match invalid ones:

52-355/9984

This regex will match all correct string, but quite long. I break it to multi line for easy reading:

^(\d{2}-?\d{3}-?\d{4})|
(\d{2}\.?\d{3}\.?\d{4})|
(\d{2}/?\d{3}/?\d{4})|
(\d{2} ?\d{3} ?\d{4})$

Can you suggest me a shorter regex that meet the requirement?

Possible duplicate of [match the same unknown character multiple times](https://stackoverflow.com/questions/11460397/match-the-same-unknown-character-multiple-times) — Aran-Fey, Nov 09 '17 at 13:34

Wiktor Stribiżew · Accepted Answer · 2017-11-09T13:41:36.100

3

You may capture the separator and use a backreference instead of repeating the pattern

^\d\d([-./ ]?)\d{3}\1\d{4}$
     ^       ^     ^^

See the regex demo

In C#:

var isValid = Regex.IsMatch(s, @"^\d\d([-./ ]?)\d{3}\1\d{4}$");

Pass the RegexOptions.ECMAScript option to the regex compiler if you only want to match ASCII digits with \d (that, by default in .NET regex, matches all Unicode digits).

Pattern details

^ - start of string
\d\d - any 2 digits
([-./ ]?) - Group 1 capturing 1 or 0 -, ., / or space
\d{3} - any 3 digits
\1 - the same value as captured in Group 1
\d{4} - any 4 digits
$ - end of string (or you might want to use \z to ensure the exact end of the string, but it is not necessary in the majority of cases).

edited Nov 09 '17 at 13:41

answered Nov 09 '17 at 13:31

Wiktor Stribiżew

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If *shortest* is really a priority, then `\d\d` is shorter than `\d{2}` – BurnsBA Nov 09 '17 at 13:36
Look great! I don't know there have a feature `\1` to match backreference – NoName Nov 09 '17 at 13:39
@BurnsBA True, `\d\d` is one char shorter than `\d{2}`. Thanks for noticing. – Wiktor Stribiżew Nov 09 '17 at 13:40

Use regex to match string in given condition

[Edit] Note:

Requirement:

Conditions:

Example

1 Answers1