const regex = /(<p value="id">(?=1445).+\R.+<p value="IP">)(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})(.+)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(<p value="id">(?=1445).+\\R.+<p value="IP">)(\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3})(.+)', 'gm')
const str = ` <list name="ItemList">
<item>
<p value="id">1445</p>
<p value="IP">10.25.30.19</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1499</p>
<p value="IP">10.40.50.53</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1445</p>
<p value="IP">10.45.70.19</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1446</p>
<p value="IP">10.77.90.55</p>
<p value="Prflen">26</p>
</item>`;
const subst = `\110.25.30.19\3`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions