{"id":3141,"date":"2021-11-29T10:15:38","date_gmt":"2021-11-29T10:15:38","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3141"},"modified":"2023-06-01T00:55:15","modified_gmt":"2023-06-01T00:55:15","slug":"python-regex-anchors","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-anchors\/","title":{"rendered":"Python Regex Anchors"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll learn how to use regular expression anchors to match the character positions including the beginning and the end of a string.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='introduction-to-the-regex-anchors'>Introduction to the regex anchors <a href=\"#introduction-to-the-regex-anchors\" class=\"anchor\" id=\"introduction-to-the-regex-anchors\" title=\"Anchor for Introduction to the regex anchors\">#<\/a><\/h2>\n\n\n\n<p><a href=\"https:\/\/www.pythontutorial.net\/python-regex\/python-regular-expressions\/\">Regular expressions<\/a> provide you with two anchors that match the positions of characters:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>^<\/code> &#8211; the caret anchor matches at the beginning of a string.<\/li>\n\n\n\n<li><code>$<\/code> &#8211; the dollar anchor matches at the end of a string.<\/li>\n<\/ul>\n\n\n\n<p>The following example uses the <code>\\d\\d<\/code> to match two digits in a time string:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-1\" data-shcb-language-name=\"JavaScript\" data-shcb-language-slug=\"javascript\"><span><code class=\"hljs language-javascript\"><span class=\"hljs-keyword\">import<\/span> re\n\ntime = <span class=\"hljs-string\">'12:20'<\/span>\nmatches = re.finditer(<span class=\"hljs-string\">'\\d\\d'<\/span>, time)\n<span class=\"hljs-keyword\">for<\/span> match <span class=\"hljs-keyword\">in<\/span> matches:\n    print(match.group())<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-1\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">JavaScript<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">javascript<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>It returns two matches:<\/p>\n\n\n<pre class=\"wp-block-code\"><span><code class=\"hljs\">12\n20<\/code><\/span><\/pre>\n\n\n<p>If you use the caret anchor (<code>^<\/code>), you&#8217;ll get one group which is the two digits at the beginning of the string. For example:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-2\" data-shcb-language-name=\"JavaScript\" data-shcb-language-slug=\"javascript\"><span><code class=\"hljs language-javascript\"><span class=\"hljs-keyword\">import<\/span> re\n\ntime = <span class=\"hljs-string\">'12:20'<\/span>\nmatches = re.finditer(<span class=\"hljs-string\">'^\\d\\d'<\/span>, time)\n<span class=\"hljs-keyword\">for<\/span> match <span class=\"hljs-keyword\">in<\/span> matches:\n    print(match.group())<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-2\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">JavaScript<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">javascript<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\"><span><code class=\"hljs\">12<\/code><\/span><\/pre>\n\n\n<p>Similarly, if you use the <code>$<\/code> anchor, you&#8217;ll get the last two digits because the <code>$<\/code> matches <code>\\d\\d<\/code> at the end of the time string:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-3\" data-shcb-language-name=\"JavaScript\" data-shcb-language-slug=\"javascript\"><span><code class=\"hljs language-javascript\"><span class=\"hljs-keyword\">import<\/span> re\n\ntime = <span class=\"hljs-string\">'12:20'<\/span>\nmatches = re.finditer(<span class=\"hljs-string\">'\\d\\d$'<\/span>, time)\n<span class=\"hljs-keyword\">for<\/span> match <span class=\"hljs-keyword\">in<\/span> matches:\n    print(match.group())<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">JavaScript<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">javascript<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\"><span><code class=\"hljs\">20<\/code><\/span><\/pre>\n\n\n<p>To check if a string is a time string, you can combine the caret (<code>^<\/code>) and dollar (<code>$<\/code>) anchors. For example:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" data-shcb-language-name=\"JavaScript\" data-shcb-language-slug=\"javascript\"><span><code class=\"hljs language-javascript\"><span class=\"hljs-keyword\">import<\/span> re\n\ntime = <span class=\"hljs-string\">'12:20'<\/span>\nmatches = re.finditer(<span class=\"hljs-string\">'^\\d\\d:\\d\\d$'<\/span>, time)\n<span class=\"hljs-keyword\">for<\/span> match <span class=\"hljs-keyword\">in<\/span> matches:\n    print(match.group())<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">JavaScript<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">javascript<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-5\" data-shcb-language-name=\"CSS\" data-shcb-language-slug=\"css\"><span><code class=\"hljs language-css\">12<span class=\"hljs-selector-pseudo\">:20<\/span><\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">CSS<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">css<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Note that the pattern <code>^\\d\\d:\\d\\d$<\/code> doesn&#8217;t validate the valid hour and minute. For example, it also matches the following string:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"CSS\" data-shcb-language-slug=\"css\"><span><code class=\"hljs language-css\">30<span class=\"hljs-selector-pseudo\">:99<\/span><\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">CSS<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">css<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>It&#8217;s not a valid time string because the valid hour is from 1 to 24 and the valid minute is from 00 to 59. Later, you&#8217;ll learn how to match the time string with valid values using the <a href=\"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-alternation\/\">alternation<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='summary'>Summary <a href=\"#summary\" class=\"anchor\" id=\"summary\" title=\"Anchor for Summary\">#<\/a><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Regex anchors match character positions, not the characters.<\/li>\n\n\n\n<li>The caret anchor (<code>^<\/code>) matches at the beginning of a string.<\/li>\n\n\n\n<li>The dollar anchor (<code>$<\/code>) matches at the end of a string.<\/li>\n<\/ul>\n<div class=\"helpful-block-content\" data-title=\"\">\n\t<header>\n\t\t<div class=\"wth-question\">Was this tutorial helpful ?<\/div>\n\t\t<div class=\"wth-thumbs\">\n\t\t\t<button\n\t\t\t\tdata-post=\"3141\"\n\t\t\t\tdata-post-url=\"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-anchors\/\"\n\t\t\t\tdata-post-title=\"Python Regex Anchors\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded wth-yes-btn\"\n\t\t\t>\n\t\t\t\t<svg\n\t\t\t\t\txmlns=\"http:\/\/www.w3.org\/2000\/svg\"\n\t\t\t\t\tviewBox=\"0 0 24 24\"\n\t\t\t\t\tfill=\"none\"\n\t\t\t\t\tstroke=\"currentColor\"\n\t\t\t\t\tstroke-width=\"2\"\n\t\t\t\t\tstroke-linecap=\"round\"\n\t\t\t\t\tstroke-linejoin=\"round\"\n\t\t\t\t\tclass=\"feather feather-thumbs-up block w-full h-full\"\n\t\t\t\t>\n\t\t\t\t\t<path\n\t\t\t\t\t\td=\"M14 9V5a3 3 0 0 0-3-3l-4 9v11h11.28a2 2 0 0 0 2-1.7l1.38-9a2 2 0 0 0-2-2.3zM7 22H4a2 2 0 0 1-2-2v-7a2 2 0 0 1 2-2h3\"\n\t\t\t\t\t><\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t<span class=\"sr-only\"> Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t<button\n\t\t\t\tdata-response=\"0\"\n\t\t\t\tdata-post=\"3141\"\n\t\t\t\tdata-post-url=\"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-anchors\/\"\n\t\t\t\tdata-post-title=\"Python Regex Anchors\"\n\t\t\t\tclass=\"wth-btn-rounded wth-no-btn\"\n\t\t\t>\n\t\t\t\t<svg\n\t\t\t\t\txmlns=\"http:\/\/www.w3.org\/2000\/svg\"\n\t\t\t\t\tviewBox=\"0 0 24 24\"\n\t\t\t\t\tfill=\"none\"\n\t\t\t\t\tstroke=\"currentColor\"\n\t\t\t\t\tstroke-width=\"2\"\n\t\t\t\t\tstroke-linecap=\"round\"\n\t\t\t\t\tstroke-linejoin=\"round\"\n\t\t\t\t>\n\t\t\t\t\t<path\n\t\t\t\t\t\td=\"M10 15v4a3 3 0 0 0 3 3l4-9V2H5.72a2 2 0 0 0-2 1.7l-1.38 9a2 2 0 0 0 2 2.3zm7-13h2.67A2.31 2.31 0 0 1 22 4v7a2.31 2.31 0 0 1-2.33 2H17\"\n\t\t\t\t\t><\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t<span class=\"sr-only\"> No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t<div class=\"wth-form hidden\">\n\t\t<div class=\"wth-form-wrapper\">\n\t\t\t<div class=\"wth-title\"><\/div>\n\t\t\t<textarea class=\"wth-message\"><\/textarea>\n\t\t\t<input type=\"button\" name=\"wth-submit\" class=\"wth-btn wth-btn-submit\" id=\"wth-submit\" \/>\n\t\t\t<input type=\"button\" class=\"wth-btn wth-btn-cancel\" value=\"Cancel\" \/>\n\t\t<\/div>\n\t<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>In this tutorial, you&#8217;ll learn how to use regular expression anchors to match the character positions including the beginning and the end of a string.<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":3122,"menu_order":2,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-3141","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/pages\/3141","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/comments?post=3141"}],"version-history":[{"count":0,"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/pages\/3141\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/pages\/3122"}],"wp:attachment":[{"href":"https:\/\/www.pythontutorial.net\/wp-json\/wp\/v2\/media?parent=3141"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}