{"id":50,"date":"2004-10-21T22:06:24","date_gmt":"2004-10-21T12:06:24","guid":{"rendered":"http:\/\/thekua.com\/rant\/?p=50"},"modified":"2012-04-01T10:12:21","modified_gmt":"2012-04-01T09:12:21","slug":"optimising-java-regular-expressions","status":"publish","type":"post","link":"https:\/\/thekua.com\/rant\/2004\/10\/optimising-java-regular-expressions\/","title":{"rendered":"Optimising Java Regular Expressions"},"content":{"rendered":"

Yesterday I had a bit of interesting work performancing tuning some code for our project’s current iteration. The code that required optimisation basically filtered some results in memory based on a set of user-specified criteria. The criteria contained a variety of different datatypes (boolean, numeric and string) and my suspicions were confirmed after a bit of simplistic profiling that the biggest consumer of computation time was the string pattern matching that was being performed.<\/p>\n

So here are a few tips for optimising Regular Expression usage in the Java langauge:<\/p>\n

    \n
  1. Minimise the number of times a regular expression is compiled (preferrably statically at source compilation time):<\/strong>
    Take this code as an example and note that the pattern should be moved outside of the loop:\n

    &nbsp;&nbsp;&nbsp;&nbsp;public List getMatchingValues(<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String [] extremelyLongListOfValuesForComparison, String stringToMatchOn) {<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;List listOfMatchingStrings = new ArrayList(extremelyLongListOfValuesForComparison.length);<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for (int i = 0; i < extremelyLongListOfValuesForComparison.length; i++ ) {<br\/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\/\/ !!! This pattern should be initialised outside of the loop<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Pattern pattern = Pattern.compile(stringToMatchOn + \".*\"); <br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (pattern.matcher(extremelyLongListOfValuesForComparison[i]).matches()) {<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;listOfMatchingStrings.add(extremelyLongListOfValuesForComparison[i]);<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br \/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return listOfMatchingStrings;<br \/>&nbsp;&nbsp;&nbsp;&nbsp;}<\/><\/code>\n <\/li>\n

  2. Be aware of libraries and the JDK where convenience methods compile their own Pattern instances. <\/strong>\n

    Many useful java.lang.String<\/a> operations in the JDK use it for example (and is well documented). These include matches<\/a>, replaceAll<\/a>, replaceFirst<\/a>, and split<\/a>.<\/p>\n

  3. Prefer Matcher.find() over Matcher.matches() if you can. <\/strong>\n

    Think of the situation where you are looking for two words that appear in a sentence in order. On initial thought, a regular expression like, \".*two.*words.*\"<\/code>, might jump to mind, of which you would instantiate a matcher and call the matches()<\/a> operation. We actually found that simplifying the regular expression to \"two.*words\"<\/code>, creating a matcher and then calling the find()<\/a> operation increased performance significantly.\n<\/li>\n

  4. Construct your Regular Expressions carefully when you need to ignore whitespace.<\/strong>\n

    For the above scenario, a user could enter several words to find any matches (in order) against a given string. Our initial code in constructing the string for the pattern was something like:<\/p>\n

    \/\/ replace any space with a wildcard match on any string in between<\/code>
    \nsearchPatternString = searchTermString.replace(\" \", \".*\"); <\/code><\/p>\n

    After a bit of testing though, a string like \"find&nbsp;&nbsp;me\"<\/code> (note the two spaces) resulted in a pattern that would have looked like \"find.*.*me\"<\/code>. One might expect that when the pattern was compiled, it could have been internally optimised to an equivalent pattern \"find.*me\"<\/code> but apparently did not. A better revised pattern was thus developed using \"\\s*\"<\/code> (any and all white space characters) and of course, rule number 1 was applied afterwards.<\/li>\n<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"

    Yesterday I had a bit of interesting work performancing tuning some code for our project’s current iteration. The code that required optimisation basically filtered some results in memory based on a set of user-specified criteria. The criteria contained a variety of different datatypes (boolean, numeric and string) and my suspicions were confirmed after a bit […]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[39],"class_list":["post-50","post","type-post","status-publish","format-standard","hentry","category-technical","tag-work-related"],"yoast_head":"\nOptimising Java Regular Expressions - thekua.com@rest<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/thekua.com\/rant\/2004\/10\/optimising-java-regular-expressions\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Optimising Java Regular Expressions - thekua.com@rest\" \/>\n<meta property=\"og:description\" content=\"Yesterday I had a bit of interesting work performancing tuning some code for our project’s current iteration. 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