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POJava regex dies on stack overflow: need a better version
 

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  1. POJava regex dies on stack overflow: need a better version
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    <p>I'm working on a <a href="http://github.com/cletus/jmd" rel="noreferrer">JMD (Java MarkDown)</a> (a Java port of <a href="http://code.google.com/p/markdownsharp/" rel="noreferrer">MarkDownSharp</a>) but I'm having an issue with one regex in particular. For the file <a href="http://github.com/cletus/jmd/blob/master/src/test/resources/functionality/Markdown_Documentation_Syntax.text" rel="noreferrer">Markdown_Documentation_Syntax.text</a> this regular expression dies:</p> <pre><code>private static final String BLOCK_TAGS_1 = "p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|script|noscript|form|fieldset|iframe|math|ins|del"; private static final String BLOCKS_NESTED_PATTERN = String.format("" + "(" + // save in $1 "^" + // start of line (with MULTILINE) "&lt;(%s)" + // start tag = $2 "\\b" + // word break "(.*\\n)*?" + // any number of lines, minimally matching "&lt;/\\2&gt;" + // the matching end tag "[ \\t]*" + // trailing spaces/tags "(?=\\n+|\\Z)" + // followed by a newline or end of ")", BLOCK_TAGS_1); </code></pre> <p>which translates to:</p> <pre><code>(^&lt;(p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|script|noscript|form|fieldset|iframe|math|ins|del)\b(.*\n)*?&lt;/\2&gt;[ \t]*(?=\n+|\Z)) </code></pre> <p>This pattern is looking for accepted block tags that are anchored to the start of a line, followed by any number of lines and then are terminated by a matching tag followed by a newline or a string terminator. This generates:</p> <pre><code>java.lang.StackOverflowError at java.util.regex.Pattern$Curly.match(Pattern.java:3744) at java.util.regex.Pattern$GroupHead.match(Pattern.java:4168) at java.util.regex.Pattern$LazyLoop.match(Pattern.java:4357) at java.util.regex.Pattern$GroupTail.match(Pattern.java:4227) at java.util.regex.Pattern$BmpCharProperty.match(Pattern.java:3366) at java.util.regex.Pattern$Curly.match0(Pattern.java:3782) at java.util.regex.Pattern$Curly.match(Pattern.java:3744) at java.util.regex.Pattern$GroupHead.match(Pattern.java:4168) at java.util.regex.Pattern$LazyLoop.match(Pattern.java:4357) ... </code></pre> <p>This can be dealt with by increasing the stack space for Java (defaults to 128k/400k for oss/ss IIRC) but the above expression is slow anyway.</p> <p>So I'm looking for a regex guru who can do better (or at least explain the performance problem with this pattern). The C# version is a little slow but works fine. PHP seems to have no issues with this either.</p> <p><strong>Edit:</strong> This is on JDK6u17 running on Windows 7 64 Ultimate.</p>
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    1. UScletus
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    1. COwhich JDK versions?
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      1. USbmargulies
    2. COThis is an awful use of regular expressions. Do you have to use regexes or can you recast this as a real recursive parser (either LR or recursive descent)?
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      1. POJava regex dies on stack overflow: need a better version
      1. USJim Garrison
    3. COHave you tried with `.*\n` to `.*?\n`?
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