Using preg_replace in PHP - Strange Behaviour - php

Question

I have the following:
$string = '4745518 some text 4510018 some text 4743618 4745518 some text 4510518 some text';
$newstring = preg_replace('/[1-9]{7,7}/','NEWTRANSACTION: $0',$string);
My intent is "replace all occurrences of seven digits with 'NEWTRANSACTION: ' plus those seven digits."
However, my result is:
NEWTRANS: 4745518 some text 4510018 some text NEWTRANS: 4743618 NEWTRANS: 4745518
some text 4510518 some text
In other words, it appears that only some of the seven-digit groups are being replaced. If I edit the original string, shift the seven digit groups around, those same seven digit groups get replaced. It's like only certain combinations of numbers are being marked for replacement. My actual input string is hundreds of lines long, and it really appears that random seven-digit groups are being replaced.
Can anyone see what's wrong? Thanks in advance.
=== EDIT ===
Thanks for all of the help so quickly. I would up using
/\b\d{7}\b/
and it works like a charm. I'm new to regex, so I learned a bit here -- although not realizing the missing '0' was total boneheadedness on my part.
My bad, showing 'NEWTRANSACTION: ' in the code, but showing 'NEWTRAN:' in the output. I was just typing the output, instead of copy/paste, and shortened it accidentally.
Thanks again.

Answer 1

Your code working fie after changing [1-9] to [0-9] (As your digits have 0 also at some places)
<?php
$string = '4745518 some text 4510018 some text 4743618 4745518 some text 4510518 some text';
echo $newstring = preg_replace('/[0-9]{7,7}/','NEWTRANSACTION: $0',$string);
https://eval.in/984686
Note:- A much shorter code given in a comment by #GrumpyCrouton,#kaii and #Barmar
/\b\d{7}\b/
Output:-https://eval.in/984792