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POActiveSupport::Inflector::camelize - help in understanding regex
 

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  1. POActiveSupport::Inflector::camelize - help in understanding regex
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    <h2>Short version:</h2> <p>I am having a rather hard time understanding two rather complex regular expressions in the <code>ActiveSupport::Inflector::camelize</code> method.</p> <p>This is the definition of the <code>camelize</code> method:</p> <pre><code>def camelize(term, uppercase_first_letter = true) string = term.to_s if uppercase_first_letter string = string.sub(/^[a-z\d]*/) { inflections.acronyms[$&amp;] || $&amp;.capitalize } else string = string.sub(/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/) { $&amp;.downcase } end string.gsub(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }.gsub('/', '::') end </code></pre> <p>I have some difficulty understanding:</p> <pre><code>string = string.sub(/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/) { $&amp;.downcase } </code></pre> <p>and:</p> <pre><code>string.gsub(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }.gsub('/', '::') </code></pre> <p>Please explain to me what they mean. Thank you.</p> <h1>Long version</h1> <p>This shows me trying to understand the regex and how I interpret them to mean. It would be very helpful if you could go through this and correct my mistakes.</p> <h2>For the first regex</h2> <pre><code>string = string.sub(/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/) { $&amp;.downcase } </code></pre> <p>Based on what I am seeing, <code>inflections.acronym_regex</code> is from the <code>Inflections</code> class in the <code>ActiveSupport::Inflector</code> module, and in the <code>initialize</code> method of the <code>Inflections</code> class,</p> <pre><code>def initialize @plurals, @singulars, @uncountables, @humans, @acronyms, @acronym_regex = [], [], [], [], {}, /(?=a)b/ end </code></pre> <p><code>acronym_regex</code> is assigned <code>/(?=a)b/</code>. From what I understand from <a href="http://www.ruby-doc.org/core-2.0.0/Regexp.html#class-Regexp-label-Anchors" rel="nofollow">http://www.ruby-doc.org/core-2.0.0/Regexp.html#class-Regexp-label-Anchors</a> ,</p> <pre><code>(?=pat) - Positive lookahead assertion: ensures that the following characters match pat, but doesn't include those characters in the matched text </code></pre> <p>So <code>/(?=a)b/</code> ensures that character <code>a</code> is inside the text, but we dont include character <code>a</code> inside the matched text, and what immediately follows character <code>a</code> must be character <code>b</code>. In other words, <code>"abc"</code> would match this regex, but <code>"bbc"</code> would not match this regex, and the matched text for <code>"abc"</code> would be <code>"b"</code> (instead of <code>"ab"</code>).</p> <p>So combining the value of <code>inflections.acronym_regex</code> into this regex <code>/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/</code>, I do not know which of the following two regex results:</p> <p>A. <code>/^(?:/(?=a)b/(?=\b|[A-Z_])|\w)/</code></p> <p>B. <code>/^(?:(?=a)b(?=\b|[A-Z_])|\w)/</code></p> <p>although I am thinking it is B. From what I understand, <code>(?:</code> provides grouping without capturing, <code>(?=</code> means positive lookahead assertion, <code>\b</code> matches word boundaries when outside brackets and matches backspace when inside brackets. So in english terms, regex B, when matching against a text, will find a string that begins with an <code>a</code> character, followed by a <code>b</code> character, and one of (1. backspace [whatever that may mean] 2. any uppercase character or underscore 3. any english alphabetic character, digit, or underscore).</p> <p>However, I find it strange that passing <code>upper_case_first_letter = false</code> to the <code>camelize</code> function should cause it to match a string starting with the characters <code>ab</code>, given that that does not seem to be how the <code>camelize</code> function behaves.</p> <h2>For the second regex</h2> <pre><code>string.gsub(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }.gsub('/', '::') </code></pre> <p>The regex is:</p> <pre><code>/(?:_|(\/))([a-z\d]*)/i </code></pre> <p>I am guessing that this regex will match a substring that starts with either an <code>_</code> or <code>/</code>, followed by 0 or more (upper or lowercase english alpabetic characters or digit). Furthermore, for the first group <code>(?:_|(\/))</code>, whether we match the <code>_</code> or <code>/</code>, the <code>([a-z\d]*)</code> capturing group will always be regarded as the second group. I do understand the part where the block tries to look up <code>inflections.acronyms[$2]</code> and on failure, does <code>$2.captitalize</code>.</p> <p>Since <code>(?:</code> means grouping without capturing, what is the value of <code>$1</code> when we match <code>_</code> ? Is it still <code>_</code> ? And for the <code>.gsub('/', '::')</code> portion, I am guessing that it gets applied for each match in the initial <code>gsub</code>, instead of being applied to the overall string after the outer <code>gsub</code> call is done?</p> <p>Apologies for the really long post. Please point out my errors in understanding the 2 regular expressions, or explain them in a better way if you can do it.</p> <p>Thank you.</p>
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    1. COPlease make your question concise.
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    2. CO@sawa, I could edit it such that it becomes very concise, but then would that help people who view the question in seeing where my difficulty lies in understanding the 2 regex? I think not.
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    3. COIn any case I think I will add a tldr section.
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