Regular expression understanding - Teh Forum

<body> <div id="header"> <span class="left"> <span class="back"><a href="lthread_list.php?webtag=DEFAULT"><span class="image mobile_back" title="Back"></span></a></span> <span class="image mobile_logo" title="Beehive Forum Logo"></span> </span> <ul> <li> <a class="reply_all" href="lpost.php?webtag=DEFAULT&reply_to=39068.0&return_msg=39068.12"> <span class="image mobile_reply_all"></span><span class="text">Reply to All</span> </a> </li> <li> <a class="navigation" href="#"> <span class="image mobile_navigation"></span><span class="text">Show messages</span> </a> </li> <li> <a class="main" href="#"><span class="image mobile_menu" title="Menu"></span> </a> </li> </ul> </div> <div class="menu main"> <ul> <li><a href="lthread_list.php?webtag=DEFAULT">Messages</a></li> <li><a href="lpm.php?webtag=DEFAULT">Inbox</a></li> <li><a href="lsearch.php?webtag=DEFAULT">Search</a></li> <li><a href="llogon.php?webtag=DEFAULT">Login</a></li> </ul> </div> <div class="menu navigation"><a href="lmessages.php?webtag=DEFAULT&msg=39068.1">1–10</a><a href="lmessages.php?webtag=DEFAULT&msg=39068.11">11–20</a><a href="lmessages.php?webtag=DEFAULT&msg=39068.21">21–30</a>…<a href="lmessages.php?webtag=DEFAULT&msg=39068.51">51–57</a></div> <div id="page_content"> <h3 class="thread_title"><a href="index.php?webtag=DEFAULT&amp;msg=39068.12">Regular expression understanding</a> </h3> <div id="messages" data-navigation="39068_12_57_10"> <a name="a39068_12"></a><div class="message" id="message_39068_12"> <div class="message_header"> <div class="message_from"> From: Peter (BOUGHTONP)<span class="message_time">31 Oct 2011 13:18</span> <div class="clearer"></div> </div><div class="message_to">To: Mizzy <span><span class="image post_read" title="Read: 31 Oct 2011 15:29"></span></span> <span class="message_count">12 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.12">39068.12</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.4">39068.4</a></div> <div class="message_body"> You've split the regex up wrong (the + quantifiers apply to the items which preceed them, not the items after them), and given incorrect descriptions for some parts. <span class="emoticon e_unsure" title=":/"><span class="e__">:/</span></span><br /> <br /> <div class="quotetext" id="code-"><b> code:</b></div><pre class="code">^ # start of input [\w-\.]+ # incorrect syntax. It should be [\w\-.]+ # since - needs escaping inside a char class, and . doesn't. # Matches a single alphanumeric, underscore, hyphen, or dot. # The + says "as many as possible, but at least one" @ # a literal @ sign ( # begin a capturing group # though should be a non-capturing group # but syntax is slightly more complex (?: ... ) vs ( ... ) [\w-]+ # this time the syntax _might_ work; some regex engines # will auto-escape hyphen if it's the last character, but if someone # that doesn't know regex might put another char afterwards, # it's always recommended to manually escape hyphen, to # avoid inadvertently creating an unwanted range. \. # since this . is outside the class it's been escaped. )+ # end the capturing group. # match the pattern inside the group as many times as # possible, at least once. [\w-]{2,4} # as above, but this time match 4 characters, at least 2. # this is not recommended for emails because # it will break for emails that end in .museum and similar # and also for numerical IP addresses ending in a single digit # and it allows things like ___ or --- which are not valid emails $ # end of input</pre><div class="edit_text">EDITED: 31 Oct 2011 13:19 by BOUGHTONP</div> </div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.12&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.12"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_13"></a><div class="message" id="message_39068_13"> <div class="message_header"> <div class="message_from"> From: Drew (X3N0PH0N)<span class="message_time">31 Oct 2011 13:38</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 13:56"></span></span> <span class="message_count">13 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.13">39068.13</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.11">39068.11</a></div> <div class="message_body"> <div class="quotetext" id="quote"><b>quote: </b>me</div><div class="quote"> I strongly agree <span class="emoticon e_pipe" title=":Y"><span class="e__">:Y</span></span></div><br /> <br /> You just expect disagreement <span class="emoticon e_hug" title="(hug)"><span class="e__">(hug)</span></span></div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.13&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.13"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_14"></a><div class="message" id="message_39068_14"> <div class="message_header"> <div class="message_from"> From: Drew (X3N0PH0N)<span class="message_time">31 Oct 2011 13:40</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 13:56"></span></span> <span class="message_count">14 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.14">39068.14</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.10">39068.10</a></div> <div class="message_body"> I think the problem with regex is not that it's hard as such (as you say, it's not) it's just that it's a large vocabulary and it's quite arbitrary.<br /> <br /> I can do pretty complex things with regex after a bit of reminding myself what's what. But then 20 minutes later I've forgotten it all again. That's the problem with regex <span class="emoticon e_sad" title=":("><span class="e__">:(</span></span></div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.14&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.14"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_15"></a><div class="message" id="message_39068_15"> <div class="message_header"> <div class="message_from"> From: Peter (BOUGHTONP)<span class="message_time">31 Oct 2011 14:15</span> <div class="clearer"></div> </div><div class="message_to">To: Drew (X3N0PH0N) <span><span class="image post_read" title="Read: 31 Oct 2011 14:17"></span></span> <span class="message_count">15 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.15">39068.15</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_14" target="_self">39068.14</a></div> <div class="message_body"> But it <em>doesn't</em> really have a large vocabulary. Well, not sure how you're defining vocabulary, but there's really only four or five types of things - quantifiers, character classes, positions, groups, and alternation, and none of those have more than a handful of variants.<br /> <br /> (Then, to reduce having to type {...} and [...] and (...) as much, there's shorthand quantifiers, shorthand classes, shorthand positions.)<br /> <br /> It is a bit of a pain that the syntax uses the same symbols for different meanings, but - once you understand when \ and ? mean the different things, and a few other bits - then the rest isn't so bad, and far less arbitrary than it seems on the surface.<br /> <br /> And it's also annoying that we've got at least five major programming variants (Perl/PCRE/Python/.NET/Java) which all have <em>slight</em> differences/benefits and then cut-down versions in JavaScript/grep/awk/etc.<br /> <br /> But it still doesn't deserve the bad reputation a lot of people assign it.<br /> <br /> <br /> withregardstoremembering/understanding,themostimportantthing,is<br /> notwritingregexesthatlooklikethis-becauseitdoesn'thelpanyonewith<br /> figuringoutwhat'sgoingonwhenyouremoveallformattinginformation.<br /> <br /> It's just a pity that extended/comment mode (where unescaped whitespace is ignored, and # starts a comment) is not the default one in almost all implementations, so people think they must squish it all on a single line.<div class="edit_text">EDITED: 31 Oct 2011 14:22 by BOUGHTONP</div> </div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.15&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.15"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_16"></a><div class="message" id="message_39068_16"> <div class="message_header"> <div class="message_from"> From: 99% of gargoyles look like (MR_BASTARD)<span class="message_time">31 Oct 2011 14:18</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 14:22"></span></span> <span class="message_count">16 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.16">39068.16</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.10">39068.10</a></div> <div class="message_body"> <p>Of course, you're probably right.</p> <p> </p> <p>My problem is simply that I usually turn to regex when I need to get something done (and learning it just gets in the way of doing something more interesting), rather than sitting down and taking the time to learn it properly.</p></div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.16&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.16"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_17"></a><div class="message" id="message_39068_17"> <div class="message_header"> <div class="message_from"> From: Drew (X3N0PH0N)<span class="message_time">31 Oct 2011 14:23</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 14:23"></span></span> <span class="message_count">17 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.17">39068.17</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_15" target="_self">39068.15</a></div> <div class="message_body"> <div class="quotetext" id="quote"><b>quote: </b></div><div class="quote">But it doesn't really have a large vocabulary. Well, not sure how you're defining vocabulary, but there's really only four or five types of things - quantifiers, character classes, positions, groups, and alternation, and none of those have more than a handful of variants.</div><br /> <br /> Yeah, that's grammar/syntax which I agree is pretty neat.<br /> <br /> The vocabulary isn't that large but it's quite large and that combined with its arbitrariness (i.e. everything is one character, so can't be differentiated/remembered that way and the characters often don't obviously relate to their subjects and so on - makes memorising hard) which makes it difficult.<br /> <br /> And it genuinely <i>is</i> complex when you get into back/forward references and have to worry about greediness and that kinda stuff. That's a genuine headfuck.<div class="edit_text">EDITED: 31 Oct 2011 14:24 by X3N0PH0N</div> </div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.17&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.17"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_18"></a><div class="message" id="message_39068_18"> <div class="message_header"> <div class="message_from"> From: Peter (BOUGHTONP)<span class="message_time">31 Oct 2011 14:37</span> <div class="clearer"></div> </div><div class="message_to">To: Drew (X3N0PH0N) <span><span class="image post_read" title="Read: 31 Oct 2011 14:40"></span></span> <span class="message_count">18 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.18">39068.18</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_17" target="_self">39068.17</a></div> <div class="message_body"> <p>I'm only half sure what you're on about with that middle paragraph. <span class="emoticon e_confused" title=":S"><span class="e__">:S</span></span></p> <p> </p> <p><br /> Most times when people worry about greediness, they should actually be using lazy quantifiers, or a negative character class.</p> <p> </p> <p>(If I was designing regex from scratch, I'd either make lazy the default, or have no default, so that people had to learn there are three different modes, and when each is appropriate.)</p> <p> </p> <p><br /> If you're using back references a lot, you're likely getting into the territory where a simple parser is likely the better choice (probably using a number of smaller, more basic regexes).</p><div class="edit_text">EDITED: 31 Oct 2011 14:37 by BOUGHTONP</div> </div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.18&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.18"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_19"></a><div class="message" id="message_39068_19"> <div class="message_header"> <div class="message_from"> From: 99% of gargoyles look like (MR_BASTARD)<span class="message_time">31 Oct 2011 15:11</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 15:40"></span></span> <span class="message_count">19 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.19">39068.19</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_18" target="_self">39068.18</a></div> <div class="message_body"> I am both a lazy quantifier AND a negative character class.</div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.19&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.19"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_20"></a><div class="message" id="message_39068_20"> <div class="message_header"> <div class="message_from"> From: 99% of gargoyles look like (MR_BASTARD)<span class="message_time">31 Oct 2011 15:11</span> <div class="clearer"></div> </div><div class="message_to">To: 99% of gargoyles look like (MR_BASTARD) <span><span class="image post_read" title="Read: 31 Oct 2011 15:11"></span></span> <span class="message_count">20 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.20">39068.20</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_19" target="_self">39068.19</a></div> <div class="message_body"> I am also putting off performance reviews. God the tedium! <span class="emoticon e_sad" title=":("><span class="e__">:(</span></span></div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.20&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.20"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div><a name="a39068_21"></a><div class="message" id="message_39068_21"> <div class="message_header"> <div class="message_from"> From: Mizzy<span class="message_time">31 Oct 2011 15:34</span> <div class="clearer"></div> </div><div class="message_to">To: Peter (BOUGHTONP) <span><span class="image post_read" title="Read: 31 Oct 2011 15:40"></span></span> <span class="message_count">21 of 57</span><div class="clearer"></div> </div> </div> <div class="message_links"> <a href="lmessages.php?webtag=DEFAULT&msg=39068.21">39068.21</a> In reply to <a href="lmessages.php?webtag=DEFAULT&msg=39068.12#a39068_12" target="_self">39068.12</a></div> <div class="message_body"> <p>sigh, yes I know I wasn't paying attention where I chopped up the code and I was generalising broadly, <span class="emoticon e_crying" title=":-(("><span class="e__">:-((</span></span> sorry I wasn't up to PB standard, but I didn't have time to write a dissertation <span class="emoticon e_tongue" title=":-P"><span class="e__">:-P</span></span></p> <p> </p> <p>PS have you considered a career as a QSA ? <span class="emoticon e_smile" title=":-)"><span class="e__">:-)</span></span> </p> <p> </p> <p>I still stand by regexbuddy it does a nice job of validating regex against different regex flavours.</p></div> <div class="message_footer"> <div class="message_footer_links"><a href="lpost.php?webtag=DEFAULT&reply_to=39068.21&return_msg=39068.12" class="reply"><span class="image post"></span>Reply</a></div> <div class="message_vote_form" data-msg="39068.21"> <span class="rating">0/0</span> <span class="image vote vote_down vote_down_off" title="Vote Down"></span> <span class="image vote vote_up vote_up_off" title="Vote Up"></span> </div> </div> </div></div> <div class="message_page_footer"> <ul> <li class="right_col"><form accept-charset="utf-8" method="get" action="lmessages.php" target="_self"><input type="hidden" name="webtag" id="webtag" class="bhinputtext" value="DEFAULT" dir="ltr" /><input type="hidden" name="msg" id="msg" class="bhinputtext" value="39068.22" dir="ltr" /><input type="submit" name="submit" id="keep_reading" value="Keep reading…" class="button" /></form></li> </ul> </div> </div> </body>