{"id":14263,"date":"2010-04-23T07:00:00","date_gmt":"2010-04-23T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2010\/04\/23\/why-cant-i-get-my-regular-expression-pattern-to-match-words-that-begin-with\/"},"modified":"2010-04-23T07:00:00","modified_gmt":"2010-04-23T07:00:00","slug":"why-cant-i-get-my-regular-expression-pattern-to-match-words-that-begin-with","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20100423-00\/?p=14263","title":{"rendered":"Why can't I get my regular expression pattern to match words that begin with %?"},"content":{"rendered":"

A customer asked for help writing a regular expression that, in the customer’s words, matched the string %1<\/code> when it appeared as a standalone word.<\/p>\n\n\n\n\n
Match<\/th>\nNo match<\/th>\n<\/tr>\n
%1<\/u><\/tt><\/td>\n%1b<\/tt><\/td>\n<\/tr>\n
:%1<\/u>:<\/tt><\/td>\nx%1<\/tt><\/td>\n<\/tr>\n<\/table>\n

One of the things that people often forget to do when asking a question is to describe the things that they tried and what the results were<\/a>. This is important information to include, because it saves the people who try to answer the question from wasting their time repeating the things that you already tried.<\/p>\n\n\n\n\n\n\n
Pattern<\/th>\nString<\/th>\nResult<\/th>\nExpected<\/th>\n<\/tr>\n
\\b%1\\b<\/tt><\/td>\n%1<\/tt><\/td>\nNo match<\/td>\nMatch<\/td>\n<\/tr>\n
\\b%1\\b<\/tt><\/td>\n:%1:<\/tt><\/td>\nNo match<\/td>\nMatch<\/td>\n<\/tr>\n
\\b%1\\b<\/tt><\/td>\nx%1<\/tt><\/td>\nMatch<\/td>\nNo match<\/td>\n<\/tr>\n
^..$<\/tt><\/td>\n%1<\/tt><\/td>\nMatch<\/td>\nMatch<\/td>\n<\/tr>\n<\/table>\n

That last entry was just to make sure that the test app was working, a valuable step when chasing a problem: First, make sure the problem is where you think it is. If the ^..$<\/tt> hadn’t worked, then the problem would not have been with the regular expression but with some other part of the program.\n “Is the \\b<\/tt> operator broken?”\n No, the \\b<\/tt> operator is working just fine. The problem is that the \\b<\/tt> operator doesn’t do what you think it does.\n For those not familiar with this notation, well, first you were probably confused by the \\b<\/tt> in the original question and skipped the rest of this article. Anyway, \\w<\/tt> matches A through Z (either uppercase or lowercase), a digit 0 through 9, or an underscore. (It’s actually more complicated than that, but the above description is good enough for the current discussion.) By contrast, \\W<\/tt> matches every other character. And in regular expression speak, a “word” is a maximal contiguous string of \\w<\/tt> characters. Finally, the \\b<\/tt> operator matches the location between a \\w<\/tt> and a \\W<\/tt>, treating the beginning and end of the string as an invisible \\W<\/tt>. I will stop mentioning the pretend \\W<\/tt> at the ends of the string; just mentally insert them where applicable.\n Okay, let’s go back to the original regular expression of \\b%1\\b<\/tt>. Notice that the percent sign is not one of the things which is matched by \\w<\/tt>. Therefore, in order for the \\b<\/tt> that comes before it to match, the character before the percent sign must be a \\W<\/tt>. That way, the \\b<\/tt> comes between a \\w<\/tt> and a \\W<\/tt>. The pattern \\b%1\\b<\/tt> means “A percent sign which comes after a \\w<\/tt>, followed by a 1 which comes before a \\W<\/tt>.”\n Looking at it another way, the string %1<\/tt> breaks down like this:<\/p>\n\n\n
\\W<\/tt><\/td>\nbeginning of string (virtual)<\/td>\n<\/tr>\n\\W<\/tt><\/td>\n%<\/tt><\/td>\n\\w<\/tt><\/td>\n1<\/tt><\/td>\n\\W<\/tt><\/td>\nend of string (virtual)<\/td>\n<\/table>\n

There is a \\b<\/tt> between the %<\/tt> and the 1<\/tt> and another one between the 1<\/tt> and the end of the string, but there is no \\b<\/tt> before the percent sign, because that location has \\W<\/tt> on both sides.\n The question started off on the wrong foot: You are having trouble writing a regular expression that matches a word that begins with %<\/tt> because there are no words which begin with %<\/tt><\/i>. The percent sign is not a \\w<\/tt> and therefore cannot be part of a word.\n What the customer is looking for is something more like (?<!\\w)%1\\b<\/tt>, a regular expression which means a percent sign not preceded by a \\w<\/tt>, followed by a 1 which comes before a \\W<\/tt><\/i>.<\/p>\n

The customer realized the mistake once it was pointed out. “I keep forgetting that I can’t get %<\/tt> included in \\w<\/tt> just because I want it to.” <\/p>\n

Michael Kaplan<\/a> covered this same topic some time ago<\/a><\/p>\n

. <\/p>\n","protected":false},"excerpt":{"rendered":"

A customer asked for help writing a regular expression that, in the customer’s words, matched the string %1 when it appeared as a standalone word. Match No match %1 %1b :%1: x%1 One of the things that people often forget to do when asking a question is to describe the things that they tried and […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-14263","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

A customer asked for help writing a regular expression that, in the customer’s words, matched the string %1 when it appeared as a standalone word. Match No match %1 %1b :%1: x%1 One of the things that people often forget to do when asking a question is to describe the things that they tried and […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/14263","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=14263"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/14263\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=14263"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=14263"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=14263"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}