<p>I have some characters that need to be replaced as above, but I don't know how: </p><p> Characters to replace: </p> <pre class="brush:php;toolbar:false;">first | end | <day> | | <red> | </red>| <a ###> | </> |</pre> <p><code>day => Get the current date (for example: 14)</code></p><p> <code>red => color red</code> </p><p> <code><a ###https://www.google.com/>Link</> => <a href="https://www.google.com/"> Link</></code></p> <p><code>Enter: Hello<red>Mr. Siro</red></code> </p><p> <code>Output: Hello<span style="color: red">Mr. Siro</span></code></p> <p>My chat history. </p> <p>Can you tell me how to write a generic function to check for replacement of the above tag? This is my code: </p> <p> <pre class="snippet-code-js lang-js prettyprint-override"><code>export const formatTags = (content) => { const firstTag = "<red>"; const secondTag = "</red>"; const tagsIndex = [...content.matchAll(new RegExp(firstTag, "gi"))].map( (a) => a.index ); const initialContent = content; tagsIndex.forEach((index) => { const tagContent = initialContent.substring( index firstTag.length, initialContent.indexOf(secondTag, index) ); if (firstTag === "<red>") { content = content.replaceAll( `${firstTag}${tagContent}${secondTag}`, `<span style="color: red">${tagContent || "わからない"}</span>` ); } }); return content; };</code></pre> </p> <p> <pre class="snippet-code-html lang-html prettyprint-override"><code><span :class="(msg.image || msg.file) && msg.text ? 'mt-2' : ''" v-html="msg.text" v-linkified:options="{ className: currentUserId === msg.senderId ? 'message-external-link' : '', }" /></code></pre> </p> <p>Sorry my English is not very good! </p><p> thank you all! </p>

const input = `Today is <day>th day of the month. 
I'd like <red> this text to be in red</red> 
and <blue>this to be in blue</blue>. 
Testing the links <a ###https://www.google.com/>with Google</>
and <a ###https://stackoverflow.com/>Stak Overflow</>`

function convert(str) {
  const replacerRules = new Map([
    [/<day>/g, new Date().getDate()],
    [/<([^>]+)>([^<]+)<\/>/g, (_, p1, p2) => `<span style="color: ${p1}">${p2}</span>`],
    [/<a #+([^>]+)>([^<]+)<\/>/g, (_, p1,p2) => `<a href="${p1}">${p2}</a>`]
  ])
  
  for (const [key, replacer] of replacerRules)
    str = str.replace(key, replacer)
  
  return str
}

console.log( convert(input) )
document.querySelector("div").innerHTML = convert(input)

<div />