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copied!<p><code>"(?=(?:\d{3})++(?!\d))"</code> is a <a href="http://www.regular-expressions.info/lookaround.html" rel="nofollow noreferrer">lookahead assertion</a>.</p> <p>It means <em>"only match if followed by ((three digits that we don't need to capture) repeated one or more times (and again repeated one or more times) (not followed by a digit))"</em>. See this <a href="http://www.regular-expressions.info/brackets.html" rel="nofollow noreferrer">explanation</a> about <code>(?:...)</code> notation. It's called non-capturing group and means you don't need to reference this group after the match.</p> <p><code>"(\\G-?\\d{1,3})"</code> is the part that should actually match (but only if the above-described conditions are met).</p> <p>Edit: I think <code>+</code> must be a special character, otherwise it's just a plus. If it's a special character (and quick search suggests that <a href="http://java.sun.com/developer/technicalArticles/releases/1.4regex/" rel="nofollow noreferrer">it is in Java, too</a>), the second one is redundant.</p> <p>Edit 2: Thanks to <a href="https://stackoverflow.com/questions/10055885/this-regex-line-exceeds-my-understanding-d3-d/10056023#comment12868478_10056023">Alan Moore</a>, it's now clear. The second <code>+</code> means possessive matching, so it means that if after checking as many 3-digit groups as possible it won't find that they're not followed by a non-digit, the engine will immediately give up instead of stepping one 3-digit group back.</p>

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