Regex in PHP to strip everything except digits and decimal point and truncate string to two digits after the decimal point

What regex in PHP would strip all characters from any input string except digits and the first encountered decimal point, and truncate everything to two digits after the first encountered decimal point (if more than one is present).
The input string should be interpreted from left to right, so string such as $34.455r.4r45,45.45 should yield the following output 34.45.
Also, "any input string" means that strings with no dots, or no digits, and those having whatever other combination of characters are also possible as an input.

For example:

Input string                                Desired output
*$234.345                                   => 234.34
(9.9)                                       => 9.9
$34.455r.4r45,45.45                         => 34.45
2023-05-29 03:40:11Z, License: CC BY-SA 4.0 => 202305290340114.0
9.95,6.432,0.3                              => 9.95
po2iaw5e.ro1i7im8jjks;fl32;i.u12ma          => 25.17
oias25.spkks                                => 25.
4545                                        => 4545
oi.as2.5.6spkks                             => .25
oi.as..spkks                                => .

I tried this expression but it doesn't work

preg_replace('/^\d+(\.\d{0,2})?$/', '', $input_string);

I'm looking for a solution that is a single regular expression and replacement string to be used in one call to preg_replace().

You can use the following preg_replace() call to replace the input with the number you are looking for:

<!-- language: php -->
preg_replace('~[^\d.]+|(.)\D*(\d?)\D*(\d?).*~', '$1$2$3', $input_string);

The regex replacement works as follow:

• The first part [^\d.]+ is trivial and deletes everything that is not a digit or a dot with the captured groups '$1$2$3'. But since you are not catching anything in that part of the regex, these capture groups are empty. So every character that is not a digit or a dot get replaced with an empty string.

• The second part (\.)\D*(\d?)\D*(\d?).* is a little bit more complicated. It has three capture groups:

  1. (\.) - An ordinary . character.
  2. (\d?) - An optional digit.
  3. (\d?) - Another optional digit.

  The remaining parts between these capture groups are for characters, which are not digits. Also the .* at the end matches the remaining of the content when you found your first dot character, so you do this replacement only once and with the whole remaining input.

Check the following test PHP script:

<?php
$cases = array();
$cases['*$234.345'] = '234.34';
$cases['(9.9)'] = '9.9';
$cases['$34.455r.4r45,45.45'] = '34.45';
$cases['2023-05-29 03:40:11Z, License: CC BY-SA 4.0'] = '202305290340114.0';
$cases['9.95,6.432,0.3'] = '9.95';
$cases['po2iaw5e.ro1i7im8jjks;fl32;i.u12ma'] = '25.17';
$cases['oias25.spkks'] = '25.';
$cases['4545'] = '4545';
$cases['oi.as2.5.6spkks'] = '.25';
$cases['oi.as..spkks'] = '.';

$regex = '~[^\d.]+|(\.)\D*(\d?)\D*(\d?).*~';
$replacement = '$1$2$3';
foreach ($cases as $input => $expected)
{
    $result = preg_replace($regex, $replacement, $input);
    $correct = $result == $expected;
    echo "Input: ".$input.", result: ".$result.", as expected: ".($correct ? 'true' : 'FALSE')."\n"."<br>";
    if (!$correct)
    {
        echo "\tShould be: ".$expected."\n";
    }
}

This will generate the following output:

<!-- language: none -->
Input: *$234.345, result: 234.34, as expected: true
Input: (9.9), result: 9.9, as expected: true
Input: $34.455r.4r45,45.45, result: 34.45, as expected: true
Input: 2023-05-29 03:40:11Z, License: CC BY-SA 4.0, result: 202305290340114.0, as expected: true
Input: 9.95,6.432,0.3, result: 9.95, as expected: true
Input: po2iaw5e.ro1i7im8jjks;fl32;i.u12ma, result: 25.17, as expected: true
Input: oias25.spkks, result: 25., as expected: true
Input: 4545, result: 4545, as expected: true
Input: oi.as2.5.6spkks, result: .25, as expected: true
Input: oi.as..spkks, result: ., as expected: true