need regex to extract two substrings from one string

Question

I have a string similar to tvm11551.iso that I am trying to parse. In this string, these bolded numbers vary: tvm 11 5 51 .iso (please ignore the spaces here). I wrote the following program in PHP that would extract those two numbers from that string:

$a = "tvm11551.iso";

if(preg_match('/^tvm\d{5}\.iso/',$a)){
    $b = preg_match('/tvm(\d\d)\d\d\d\.iso/' , $a);
    $c = preg_match('/tvm\d\d\d(\d\d)\.iso/' , $a);
    echo "B: " . $b . "<br>";
    echo "C: " . $c;
}

However, I am getting the output as:

B: 1
C: 1

How do I fix the RegEx to get the expected output?

Is it always two digits on each side of the 5, and 5 digits in total? — Michael Berkowski
– Michael Berkowski, Commented Aug 1, 2012 at 20:50

Michael Berkowski · Accepted Answer · 2012-08-02 02:14:25Z

5

The 1 you are seeing as output is the number of matches, not their contents, that preg_match() returns on a successful match. Since each of your tests had only one () capture group, one match was returned. You need to capture matches into an array as the third argument to preg_match():

$a = "tvm11551.iso";
$matches = array();
preg_match('/^tvm(\d{2})5(\d{2})\.iso$/', $a, $matches);

var_dump($matches);
array(3) {
  [0] =>
  string(12) "tvm11551.iso"
  [1] =>
  string(2) "11"
  [2] =>
  string(2) "51"
}

edited Aug 2, 2012 at 2:14

answered Aug 1, 2012 at 20:52

Michael Berkowski

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3 Comments

KRyan Over a year ago

A better answer to the actual question asked (+1), but it's still pointless to be using RegEx here when a simple substring will do the job.

Rahul Desai Over a year ago

@Michael: Thanks! Thats was easy! I later realized what a stupid question I asked, lol.

apfelbox Over a year ago

@Michael Just a small hint: the output 1 is an actual one, no true, since preg_match() returns the number of matches. preg_match() only searches for the first match and stops, so the return value is either 1 or 0. docs, see "Return Values"

KRyan · Accepted Answer · 2012-08-01 20:51:49Z

3

Based on the patterns you have, I don't see any reason to be using RegEx at all, after the initial match.

You would just want this:

if(preg_match('/^tvm\d{5}\.iso/',$a)){
    $b = substr($a, 3, 5);
    $c = substr($a, 6, 8);

answered Aug 1, 2012 at 20:51

KRyan

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2 Comments

Michael Berkowski Over a year ago

The main overhead is in loading the regex engine, so if you have already done so to match the filename, it probably won't hurt to just do the captures in the regex at this point. I'd be interested to see the benchmark to know if it is faster to switch into two substr() function calls after already initializing the regex...

KRyan Over a year ago

Interesting, didn't know that about the overhead. I'd be interested in the benchmark too.

Oussama Jilal · Accepted Answer · 2012-08-01 20:54:11Z

1

preg_match returns the number of times the pattern matched (but since preg_match always stops at the first match it finds, it always returns 0 or 1). if you want to get the numbers that got matched, you haves to pass an array as the third parameter for preg_match. see preg_match documentation

answered Aug 1, 2012 at 20:54

Oussama Jilal

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