regular expression for telephone/mobile per country and variable naming

Question

would like to know if there is a better approach in doing a telephone/mobile number regular expression for specific country. this includes naming variables like how to know if format is international format (is it with + sign?), a national format (is it the area code enclosed in parenthesis) or a local format (no + sign, and no area code), currently they are labelled as country_name_telephone/country_name_mobile or country_name_telephone_N_digit, etc...

below are the regular expression for some country.

/** matches the following pattern:
 * - 01-111-11-11
 * - 01 111 11 11
 * - 011-11-11-11
 * - 011 11 11 11
 * - 011111111
 */
public final static String CTRY_BELGIUM_TELEPHONE = "^0(\\d{8}|\\d\\s\\d{3}\\s\\d\\d\\s\\d\\d|\\d-\\d{3}-\\d\\d-\\d\\d|\\d\\d\\s\\d\\d\\s\\d\\d\\s\\d\\d|\\d\\d-\\d\\d-\\d\\d-\\d\\d)$";
/** matches the following pattern:
 * - 0412-34-56-78
 * - 0412 34 56 78
 * - 0412-345-678
 * - 0412 345 678
 * - 0412345678
 */
public final static String CTRY_BELGIUM_MOBILE = "^04\\d\\d(\\d{6}|\\s\\d\\d\\s\\d\\d\\s\\d\\d|\\s\\d{3}\\s\\d{3}|-\\d\\d-\\d\\d-\\d\\d|-\\d{3}-\\d{3})$";
/** matches the following pattern:
 * - 0000-0000
 * - 0000 0000
 * - 00000000
 */
public final static String CTRY_HONG_KONG_TELEPHONE = "^(\\d{4}[-\\s]?\\d{4})$";
/** matches the following pattern:
 * - [phone]
 * - [phone]
 * - [phone]
 */
public final static String CTRY_HONG_KONG_MOBILE = "^\\+852(\\d{8}|-\\d{4}-\\d{4}|\\s\\d{4}\\s\\d{4})$";
/** matches the following pattern:
 * - 212-0000
 * - 212 0000
 * - 2120000
 */
public final static String CTRY_UNITED_STATES_TELEPHONE_7_DIGIT = "^[2-9]((?!11)\\d{2})[-\\s]*\\d{4}$";
/** matches the following pattern:
 * - 200 212 0000
 * - 200-212-0000
 * - 2002120000
 */
public final static String CTRY_UNITED_STATES_TELEPHONE_10_DIGIT = "^[2-9]\\d{2}((-[2-9]((?!11)\\d{2})-)|(\\s[2-9]((?!11)\\d{2})\\s)|([2-9]((?!11)\\d{2})))\\d{4}$";
/** matches the following pattern:
 * - all matches in CTRY_UNITED_STATES_TELEPHONE_7_DIGIT and CTRY_UNITED_STATES_TELEPHONE_10_DIGIT
 */
public final static String CTRY_UNITED_STATES_TELEPHONE = "("+CTRY_UNITED_STATES_TELEPHONE_7_DIGIT+")|("+CTRY_UNITED_STATES_TELEPHONE_10_DIGIT+")";
/** matches the following pattern:
 * - [phone]
 * - [phone]
 * - [phone]
 */
public final static String CTRY_UNITED_STATES_MOBILE = "^\\+1((-[2-9]\\d{2}-[2-9]((?!11)\\d{2})-)|(\\s[2-9]\\d{2}\\s[2-9]((?!11)\\d{2})\\s)|([2-9]\\d{2}[2-9]((?!11)\\d{2})))\\d{4}$";

Answer 1

For CTRY_BELGIUM_TELEPHONE:

First, to understand what the regex is doing, I'll add the railroad diagram

RegEx RailRoad Diagram

Demo on RegEx101

What it matches:

Zero followed by

1. Eight Digits
2. Digit-Space-Three_Digits-Space-Two_Digits-Space-Two_Digits
3. Digit-Hyphen-Three_Digits-Hyphen-Two_Digits-Hyphen-Two_Digits
4. Two_Digits-Space-Two_Digits-Space-Two_Digits-Space-Two_Digits
5. Two_Digits-Hyphen-Two_Digits-Hyphen-Two_Digits-Hyphen-Two_Digits

Now, looking at above, we can easily spot that many of the things are use multiple times in the same sub-pattern.

Can we combine/reuse them?

For simplicity, I'll use single backslash from here onward. You need to escape it by prepending another backslash to escape it in string.

When I say nth road, it'll refer to the nth path in the above diagram.

Review:

The first road is straight forward. Match eight digits. Simple!

The second road, \d\s\d{3}\s\d\d\s\d\d. As you've used \d{3}, similarly we can write \d\d as \d{2}. Cool!

It'll now become \d\s\d{3}\s\d{2}\s\d{2}.

The third road similarly can be written as \d-\d{3}-\d{2}-\d{2}.

For fourth and fifth road, same can be used.

Second and Third roads are similar except the separator used between digits. Is there any way to combine them?

Back-reference.

How can back-reference be used here?

I'll show you: The character should be captured in capturing group and refer it with back-reference whenever needed.

\d([\s-])\d{3}1円\d{2}1円\d{2}

The regex here can be used to match both second and third road.

1. \d: Match a digit
2. ([\s-]): Match either a space character or hyphen and add it into first capturing group
3. \d{n}: Match digits n times.
4. 1円: Match whatever is in first captured group(see #2 above)

There still is repetition in 1円\d{2}1円\d{2}. This can be added in group and repeated twice (?:1円\d{2}). Since, back-reference is used, it's not better to use non-capturing group.(You'll know why in the 4th and 5th road).

Similarly, for fourth and fifth road, the regex will be

\d{2}([\s-])\d{2}(?:1円\d{2}){2}

At this stage, the regex will become

^0(?:\d{8}|\d([\s-]?)\d{3}(?:1円\d{2}){2}|\d{2}([\s-])\d{2}(?:2円\d{2}){2})$

The first part \d{8} can also be removed by using ? quantifier on the first captured group. By doing this, we're making the [\s-] optional and thus 1円 will be empty and will match eight digits with no spaces between them.

So, Final RegEx is

/^0(?:\d([\s-]?)\d{3}(?:1円\d{2}){2}|\d{2}([\s-])\d{2}(?:2円\d{2}){2})$/

Here's railroad diagram for this

New RegEx Railroad

Note that 1 and 2 in the diagram refer to respective capturing group.

Demo on RegEx101

Although, this answer only explains about one RegEx, other RegExes can be modified in the similar way.

• vims liu: Hi, Thank you very much for the detailed explanation, and for introducing the Back-reference, the other regex can greatly improve.
• vims liu: Out of curiosity, from which site did you generate that railroad diagram? cannot find it in regex101.
• Tushar: @vims I've used atom plugin to generate diagram.