如何计算匹配的字符数？

Question

I'm trying to replace a sequence of numbers with zero. Always want to keep first digit and replace all the rest with zero. Something like this:

$numb    = 4124;
$newnumb = 4000;  // what I want

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Note: Sometimes there is a float number like this 212.1 . For float numbers I don't match float part. So in 212.1 , I just match 212 .

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Here is my pattern:

^(\d)([^\.]+)

Now $1 contains first digit, and I want to know how can I put 0 instead of the rest of the digits?

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Examples:

423.13 => 400
1232   => 1000
99.123 => 90

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How can I do that using regex?

Answer 1

As you already have the prefect regex ^(\\d)([^\\.]+) you just need to use preg_replace_callback() and use the amount of characters in the second capturing group for the amount of 0's you want to print with str_repeat() , eg

echo preg_replace_callback("/^(\d)([^\.]+)\..*/", function($m){
    return $m[1] . str_repeat(0, strlen($m[2]));
}, $string);

Answer 2

You can use the \\G anchor:

echo preg_replace('/(?:\G(?!\A)|\d)\K\d(?:\.\d*)?/S', '0', $num);

details:

(?:
    \G        # position after the previous match 
    (?!\A)    # (but not at the start of the string)
  |           # OR
    \d        # first digit (you can also check if there is no leading dot)
)
\K            # start the match result at this position
              # (to preserve the first digit)
\d
(?:\.\d*)? # eventual decimals (you can change * to + to avoid to
           # remove a dot that ends a sentence)

More efficient way (with \\d in factor at the beginning) :

echo preg_replace('/\d(?:(?<=\G.)|\K\d)(?:\.\d+)?/', '0', $num);