Php - how to replace roman numerals in a string with integers?

Question

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I have a string that contains words and one or more roman numerals from one (I) to ten (X).
I need to replace those numerals with integers.

This is what I tried:

PHP

$reps = [
'I' => 1,
'II' => 2,
'III' => 3,
'IV' => 4,
'V' => 5,
'VI' => 6,
'VII' => 7,
'VIII' => 8,
'IX' => 9,
'X' => 10
];

$str = "word word VII word ...";

$str2 = str_replace(array_keys($reps), $reps, $str);

But it doesn't work: "VII" is replaced by "511" instead of "7".

How can I fix it?

What I have tried:

PHP

$reps = [
'I' => 1,
'II' => 2,
'III' => 3,
'IV' => 4,
'V' => 5,
'VI' => 6,
'VII' => 7,
'VIII' => 8,
'IX' => 9,
'X' => 10
];

$str = "word word VII word ...";

$str2 = str_replace(array_keys($reps), $reps, $str);

Posted 5-Feb-24 11:57am

LB2371

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3 solutions

Solution 2

As documented at PHP: str_replace - Manual[^], str_replace() matches from first to last.
To do what you want, you need to put all search strings BEFORE any prefixes of them, so an order like

IV IX III II I VIII VII VI V X

should work.

Posted 5-Feb-24 13:28pm

Peter_in_2780

Comments

CPallini 6-Feb-24 2:32am

I bet it does.

Peter_in_2780 6-Feb-24 4:55am

:) Thanks, Carlo.

LB2371 6-Feb-24 2:55am

Isn't there a way to treat each roman numeral as a word and do a whole-word replacement? Maybe using "preg_replace" or "preg_replace_callback"?

Peter_in_2780 6-Feb-24 4:54am

Yes that would work, and so would including a trailing space in the match string if you want to stick with str_replace.
Your choice really comes down to what you want to do with edge cases, like a "word" starting with I, or the roman number at the start or end of input, and so on.

LB2371 10-Feb-24 5:33am

Your "empirical" solution doesn't work for me: for example, "VII" is replaced by "52" (instead of "7"). See my own solution below.

Solution 3

In addition to the above and your comment -

Your problem is that 'str_replace' performs replacements in the order they appear in your array as mentioned above. You can use the 'strtr' function (PHP strtr Function [^]) along with a custom replacement function to return the correct replacement, tested with all array items in W3Schools Editor [^] -

HTML

<!DOCTYPE html>
<html>
<body>

<?php
$reps = [
    'I' => 1,
    'II' => 2,
    'III' => 3,
    'IV' => 4,
    'V' => 5,
    'VI' => 6,
    'VII' => 7,
    'VIII' => 8,
    'IX' => 9,
    'X' => 10
];

$str = "word word IX word ...";

function replaceNumerals($matches) {
    global $reps;
    return $reps[$matches[0]];
}

$str2 = preg_replace_callback('/\b(?:' . implode('|', array_map('preg_quote', array_keys($reps))) . ')\b/', 'replaceNumerals', $str);

print_r($str2);
?>

</body>
</html>

Returns 'word word 9 word ...'

Posted 6-Feb-24 7:07am

Andre Oosthuizen

Comments

LB2371 6-Feb-24 13:19pm

Where's the "strtr" function in your code?

Andre Oosthuizen 7-Feb-24 15:24pm

If you follow the link provided you will notice that I have modified the code a little to conform to that will work for you...

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LB2371 · Accepted Answer · 2024-02-09T23:35:00

PHP

$nums = ['I', 'II', 'III', 'IV', 'V', 'VI', 'VII', 'VIII', 'IX', 'X'];
$str = preg_replace_callback('/\b(I|II|III|IV|V|VI|VII|VIII|IX|X)\b/', fn($m) => array_search($m[0], $nums) +1, $str);

// $str = preg_replace_callback('/\b('.implode('|', $nums).')\b/', fn($m) => array_search($m[0], $nums) +1, $str);