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  1. PO
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    <p>First of all, you use pattern matching where you don’t have a <em>pattern</em>. This is not only wasting performance, it is an error. For <code>"://www."</code> used as a pattern the last dot matches <em>any character</em>.</p> <p>If you want to replace a simple <code>String</code> rather than a pattern, you can use <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#replace%28java.lang.CharSequence,%20java.lang.CharSequence%29" rel="nofollow"><code>String.replace</code></a> instead of <code>String.replaceAll</code>. This fixes the error and saves the entire overhead of creating a <code>Pattern</code> and a <code>Matcher</code> internally.</p> <p>If you want to use the regex matcher, the simplest fix is to use <code>Pattern.compile("://www.", Pattern.LITERAL)</code> to stop interpreting special characters and allow using the Boyer–Moore algorithm for the entire sequence. This, however, will only pay off if you can store and re-use the prepared pattern.</p> <p>Further, the <code>replaceAll</code> method offers the feature of interpreting back-references to groups of the match in the replacement <code>String</code>. Since you’re not using this feature, you can save the overhead associated with that feature by implementing the replacement loop by yourself. As a bonus you can use <code>StringBuilder</code> where the <code>Matcher</code> uses <code>StringBuffer</code>, but that’s only a minor improvement.</p> <pre><code>static String replace(String source) {// the pattern would be better off being re-usable stored in a static field final Pattern pattern = Pattern.compile("://www.", Pattern.LITERAL); final Matcher m = pattern.matcher(source); boolean result = m.find(); if (result) { StringBuilder sb = new StringBuilder(source.length()+16); int p=0; do { sb.append(source, p, m.start()).append("://www-x1."); p=m.end(); } while (m.find()); sb.append(source, p, source.length()); return sb.toString(); } return source; } </code></pre> <p>But as already said, not using regex at all might be the better choice for this simple case here.</p>
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    1. POJava Performance optimise replacing one String with another, repeatedly
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    1. POJava Performance optimise replacing one String with another, repeatedly
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    1. COI saw afterwards, that you referred to `replace` rather than `replaceAll` in your question. But were you aware of the difference? Moving from `replace` to regex is not a natural choice…
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    2. COI have no clue about the difference in overhead, I must admit. So you would recommend using the simplest solution, which is the String `replace` method? Or is the difference marginal? I do declare the Pattern objects as static final fields.
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    3. COI would stay at the simplest solution, `String.replace`. The pattern example of my answer would pay off for larger search strings only, but `"://www."` is rather short. If in doubt, benchmark both variants.
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