Regular expression for unlimited digits but limited other character

Question

I'm trying to find a regular expression for unlimited digits but limited underscores.

So let's say max_underscore = 3.

Then

123_456_678 is ok (3 underscores)
1234213_2132135678 is ok (2 underscores)

But

13_4156_7_012_1 is not ok (4 underscores)

Possible duplicate of [Learning Regular Expressions](http://stackoverflow.com/questions/4736/learning-regular-expressions) — Sayse, Feb 02 '17 at 13:38
FWIW, you don't need regex, most languages have a way to find number of occurances of a character in a string. — Sayse, Feb 02 '17 at 13:39
Then you should try it yourself first and then include your attempts in your question. — Sayse, Feb 02 '17 at 13:42
Is it allowed to have underscore at the begining or at the end of the string? Is it allowed to have 2 or more underscores consecutive? — Toto, Feb 02 '17 at 13:55

Toto · Accepted Answer · 2017-02-02T14:15:57.113

If underscore is not allowed at the begining or at the end and it is not allowed to have 2 or more consecutive underscore:

^(?:\d+_?){3}\d+$

If underscore is allowed at the begining or at the end:

^(?:\d*_?){3}\d*$

These regex match string that have 0 upto 3 underscore but not more.

See these 2 regexes in action:

$strings = array(
'123456',
'1_2_3_456',
'123_456',
'_123_456_',
'1_2_3_4_5_6',
'123___456',
);
$num = 3;
echo 'using "^(?:\d+_?){3}\d+$"',"\n";
foreach($strings as $string) {
    if (preg_match("/^(?:\d+_?){".$num."}\d+$/", $string)) {
        echo "OK: $string\n";
    } else {
        echo "KO: $string\n";
    }
}
echo 'using "^(?:\d*_?){3}\d*$"',"\n";

foreach($strings as $string) {
    if (preg_match("/^(?:\d*_?){".$num."}\d*$/", $string)) {
        echo "OK: $string\n";
    } else {
        echo "KO: $string\n";
    }
}

Output:

using "^(?:\d+_?){3}\d+$"
OK: 123456
OK: 1_2_3_456
OK: 123_456
KO: _123_456_
KO: 1_2_3_4_5_6
KO: 123___456
using "^(?:\d*_?){3}\d*$"
OK: 123456
OK: 1_2_3_456
OK: 123_456
OK: _123_456_
KO: 1_2_3_4_5_6
OK: 123___456

Aaron · Answer 2 · 2020-12-30T18:57:33.710

1

If you can, represent each of the other characters.

For example with 3 underscores :

\d*_\d*_\d*_\d*

If you want to match 3 or less underscores, you can make parts of the regex optional :

\d*(_\d*(_\d*(_\d*)?)?)?

Note that this allow two consecutive underscores and underscores at the start and end of the string, which you could avoid by using + instead of * :

\d+_\d+_\d+_\d+

edited Dec 30 '20 at 18:57

answered Feb 02 '17 at 13:38

Aaron

24,009
2
33
57

Doesn't match string with less than 3 underscore as requested. – Toto Feb 02 '17 at 14:06

Regular expression for unlimited digits but limited other character

2 Answers2