Introduction to Regular expressions and common usage

<span id="Label3"></p><p class="MsoNormal"><p class="MsoNormal">Regular Expressions <span>(Regular Expression)</span><span>, also known as formal representations and regular representations, are often used in Real-world software development Projects. It uses a single string to describe, match, and obtain a series of results that conform to a certain syntactic rule. </span></p></p><p class="MsoNormal"><p class="MsoNormal">I will write this <b>regular expression</b> <b>tutorial</b> for you from the most basic part.</p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal"><b>The origin of regular expressions</b><b></b></p></p><p class="MsoNormal"><p class="MsoNormal"><b></b></p></p><p class="MsoNormal"><p class="MsoNormal">1956 <span>, mathematician</span> <span>Stephen Kleene</span> <span>in</span> <span>Warren McCulloch</span> <span>and</span> <span>Walter Pitts</span> <span> Based on the work of the early nervous system, a mathematical symbolic system</span> <span>--regular sets (</span> <span>set of rules</span> <span>)</span><span>was designed, which was quickly used by computer scientists for scanning or lexical analysis of Compilers. The powerful text processing capability of regular expressions is quickly applied to</span> the <span>Unix</span> <span>tool-software</span> <span>grep</span> <span>; Since then, regular expressions have been widely</span> <span>used in Unix</span> <span>system,</span><span>Perl</span><span></span>,<span>PHP</span><span>,</span><span>Delphi</span><span>,</span> <span>JAVAScript</span> <span>,</span> <span>C # (. NET)</span> <span>,</span> <span>JAVA</span> <span>,</span> Python,<span>Ruby</span> , <span>and other languages and development environments. </span></p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal"><b>Regular expression definitions</b><b></b></p></p><p class="MsoNormal"><p class="MsoNormal"><b></b></p></p><p class="MsoNormal"><p class="MsoNormal">A regular expression is a language used to describe a specific structure <span>(</span> <span>rule</span> <span>)</span> <span>of a string that is executed by the relevant ENGINE. The regular expression is visualized in Description</span> <span>1</span> <span>. </span></p></p><p class="MsoNormal"><p class="MsoNormal"><span><br></span></p></p><p class="MsoNormal"><p class="MsoNormal"><span></span></p></p><p class="MsoNormal"><p class="MsoNormal">Figure <span>1</span> <span>Description of the visualization of the regular expression</span></p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal"><b>The role of regular expressions</b><b></b></p></p><p class="MsoNormal"><p class="MsoNormal"><b></b></p></p><p class="MsoNormal"><p class="MsoNormal">1. <span>Data Validation</span></p></p><p class="MsoNormal"><p class="MsoNormal">Tests the input string, whether it conforms to a certain rule, whether the input is allowed, and so On. For example, you can verify <span>email</span> <span>address legality, url, phone number, date of birth, and so On. </span></p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal">2. <span>manipulating text</span></p></p><p class="MsoNormal"><p class="MsoNormal">Used to identify specific text in a document, delete the text completely, or replace it with other text or Characters.</p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal">3. <span>extracting substrings</span></p></p><p class="MsoNormal"><p class="MsoNormal">Based on pattern matching, you can find specific text within a document or input fields, and often need to be extracted first when involved in a replacement operation.</p></p><p class="MsoNormal"><p class="MsoNormal"></p></p><p class="MsoNormal"><p class="MsoNormal"><b>Regular expression Basic syntax</b><b></b></p></p><p class="MsoNormal"><p class="MsoNormal"><b></b></p></p><p class="MsoNormal"><p class="MsoNormal">1. <span>Direct Volume characters</span> <span>(</span> <span>usually invisible characters and characters that match themselves</span> <span>)</span></p></p><p class="MsoNormal"><p class="MsoNormal">2. <span>character Classes</span> <span>(</span> <span>can match multiple characters</span> <span>)</span></p></p><p class="MsoNormal"><p class="MsoNormal">3. <span>Repeat</span></p></p><br></span> <p><p></p></p> <p class="MsoNormal"><p class="MsoNormal"><span></span></p></p> <p class="MsoNormal"><p class="MsoNormal">4. <span>select, group, and anchor characters</span></p></p> <p class="MsoNormal"><p class="MsoNormal">5. <span>logo</span> <span>(</span> <span>indicating the operating mode of the engine</span> <span>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal"><span></span></p></p> <p class="MsoNormal"><p class="MsoNormal">Description</p></p> <p class="MsoNormal"><p class="MsoNormal">Use <span>the "(? i)"</span> indicator before the regular expression <span>, for example:</span><span>(? i) ^root$</span><span>,</span> <span>root</span><span></span>,<span>root</span><span>, </span> <span>ROOT</span> <span>meets the Requirements. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">6. <span>Other</span></p></p> <p class="MsoNormal"><p class="MsoNormal">1) <span>match meta-character</span> <span>"([{\ ^ $ |)?" * +. "</span> <span>, need to use</span> <span>"\"</span> <span>Escaped. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"><span>for example: match</span> <span>"."</span> <span>, the regular expression is</span> <span>"\."</span> <span>. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">2) <span>greedy quantifier and lazy quantifier</span></p></p> <p class="MsoNormal"><p class="MsoNormal">The lazy quantifier only adds a <span>"?"</span> to the greedy quantifier. <span>. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">When a greedy quantifier is used to match, it first treats the whole string as a match, if the match succeeds, if it does not match, the last character is truncated, and if it does not match, the last character is truncated to match until a character match is Reached.</p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">When matching with an inert quantifier, it first treats the first character as a match, exits if successful, or, if it fails, the first two characters of the test, incremented until a suitable match is Encountered.</p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">For example:<span>"\d+"</span> <span>is a greedy quantifier, and</span> <span>"\d+?"</span> <span>Non-greedy</span> <span>(</span> <span>inert</span> <span>)</span> <span>Quantifiers. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">3) <span>Sub-match</span></p></p> <p class="MsoNormal"><p class="MsoNormal">The internal grouping matches, using <span>"()"</span> to <span>mark a grouping. </span></p></p> <p class="MsoNormal"><p class="MsoNormal">Each subgroup of a sub-match is placed in a special place for future use, and these stored values are special values in the grouping, called reverse References.</p></p> <p class="MsoNormal"><p class="MsoNormal">For example: Verify that the input is a date and then extract it to the month with the regular expression <span>"^\d{4}\-(\d{2}) \-\d{2}$"</span><span>. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">4) forward <span>looking and negative forward looking</span></p></p> <p class="MsoNormal"><p class="MsoNormal">Forward forward:<span>(? <=</span> <span>character</span> <span>)</span> <span>or</span> <span>(? =</span> <span>character</span> <span>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">Note: Be sure to have an equal LINE.</p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">Negative outlook:<span>(? <!</span> <span>character</span> <span>)</span> <span>or</span> <span>(?!</span> <span>character</span> <span>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">Note: Be sure to have a non-equal line.</p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">That is, we can set ourselves where the matching boundaries are, which is often used in string Extraction.</p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">Example:</p></p> <p class="MsoNormal"><p class="MsoNormal">Example <span>1</span><span>, we take</span> the characters before <span>"#"</span> , <span>but do not include</span> <span>"#"</span><span>, its regular expression:</span><span>[\w]+ (? = #)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">Example <span>2</span><span>, we take</span> a character that is not before <span>"#</span> ", <span>but does not include</span> <span>"#"</span><span>, its regular expression: </span> <span>[\w]+ ( ?! #)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">Example <span>3</span><span>, we take</span> the characters between <span>"<></span> ", <span>but do not include</span> <span>"<>"</span><span>, its regular expression: </span> <span>(? <= <) [\w]+ (?=>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"><b>Common usage of regular expressions</b><b></b></p></p> <p class="MsoNormal"><p class="MsoNormal"><b></b></p></p> <p class="MsoNormal"><p class="MsoNormal">1. <span>Digital</span></p></p> <p class="MsoNormal"><p class="MsoNormal">1) <span>Positive integer: </span> <span>^[1-9][0-9]*$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">2) <span>non-positive integers: </span> <span>^ ((-[1-9][0-9]*) | ( 0)) $</span></p></p> <p class="MsoNormal"><p class="MsoNormal">3) <span>negative integer:</span><span>^-[1-9][0-9]*$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">4) <span>integer: </span> <span>^ (0|-?[ 1-9][0-9]*) $</span></p></p> <p class="MsoNormal"><p class="MsoNormal">5) <span>non-negative floating point number:</span><span>^\d+ (\.\d+)? $</span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">2. <span>Letters</span></p></p> <p class="MsoNormal"><p class="MsoNormal">1) <span>English string:</span><span>^[a-za-z]+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">2) <span>English capitalization string:</span><span>^[a-z]+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">3) <span>English lowercase string:</span><span>^[a-z]+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">4) <span>English characters number string:</span><span>^[a-za-z0-9]+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">5) <span>English numerals plus underline string:</span><span>^\w+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">3. <span>Other</span></p></p> <p class="MsoNormal"><p class="MsoNormal">1.e-mail <span>address:</span><span>^[\w-]+ (\.[ \w-]+) *@[\w-]+ (\.[ \w-]+) +$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">2.URL<span>:</span><span>^http:\/\/[a-za-z0-9]+\.[ a-za-z0-9]+[\/=\?%\ -&_~ ' @[\]\ ': +!] * ([^<>\ "\"]) *$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">3. <span>zip code:</span><span>^[1-9]\d{5}$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">4. <span>chinese:</span><span>^[\u4e00-\u9fa5]+$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">5. <span>Phone number:</span><span>^ ((</span>\d2,3) | ( \d{3}\-))? (0\d2,3|0\d{2,3}-)? [1-9]\d{6,7} (\-\d{1,4})? $</p></p> <p class="MsoNormal"><p class="MsoNormal">6. <span>Mobile Number:</span><span>^1\d{10}$</span></p></p> <p class="MsoNormal"><p class="MsoNormal">7. Leading and <span>trailing spaces:</span><span>(^\s+) | ( \s+$)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">8. <span>Identity card:</span><span>^ (\d{15}|\d{18}) $ (</span> <span>note: China's identity card is a</span> <span></span> level <span>or</span> <span></span> <span></span> a <span>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal">9. <span>Account Number:</span><span>^[a-za-z]\w{4,15}$ (</span> <span>note: The letter begins with the Allow</span> <span>5-16</span> <span>byte, allowing alphanumeric</span> underlines <span>) </span></p></p> <p class="MsoNormal"><p class="MsoNormal">10.IP<span>:</span><span>^ ([1-9]\d{0,1}|1\d{2}|2[0-4]\d|25[0-5]) (\. ( [1-9]\d{0,1}|1\d{2}|2[0-4]\d|25[0-5]) {3}$ (IP</span> <span>is a number greater than or equal</span> <span>to 0</span> <span>and less than or equal</span> to <span>255</span> <span>, </span> <span>"."</span> <span>to validate each number sub-item again and</span> <span>"."</span> <span>stitching on it is possible.</span> <span>)</span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"><b>Summarize</b><b></b></p></p> <p class="MsoNormal"><p class="MsoNormal"><b></b></p></p> <p class="MsoNormal"><p class="MsoNormal">The function of regular expression is very powerful, we can feel it when we actually use it. of course, It is not a simple matter to remember so many regular expression rules at the same time. You can record common regular expressions <span>(</span> as <span>in this article</span> <span>)</span><span>and refer to them when you need Them. </span></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal"></p></p> <p class="MsoNormal"><p class="MsoNormal">Source:<span>C</span> <span>blog</span> <span>/programmer_zhou</span> <span>'s Column</span></p></p> <br> <p><p></p></p> <br> <p><p></p></p> <p><p>Introduction to Regular expressions and common usage</p></p>