{"id":108470,"date":"2020-02-01T14:11:45","date_gmt":"2020-02-01T14:11:45","guid":{"rendered":"https:\/\/blog.finxter.com\/?p=5946"},"modified":"2020-02-01T14:11:45","modified_gmt":"2020-02-01T14:11:45","slug":"python-regex-how-to-match-the-start-of-line-and-end-of-line","status":"publish","type":"post","link":"https:\/\/sickgaming.net\/blog\/2020\/02\/01\/python-regex-how-to-match-the-start-of-line-and-end-of-line\/","title":{"rendered":"Python Regex \u2013 How to Match the Start of Line (^) and End of Line ($)"},"content":{"rendered":"

This article is all about the start of line ^ and end of line $ regular expressions in Python’s\u00a0re library<\/a>. <\/strong>These two regexes are fundamental to all regular expressions—even outside the Python world. So invest 5 minutes now and master them once and for all!<\/p>\n

Python Re Start-of-String (^) Regex<\/h2>\n

You can use the caret operator ^ to match the beginning of the string. For example, this is useful if you want to ensure that a pattern appears at the beginning of a string. Here’s an example:<\/p>\n

>>> import re\n>>> re.findall('^PYTHON', 'PYTHON is fun.')\n['PYTHON']<\/pre>\n
The findall(pattern, string) method finds all occurrences of the pattern in the string. The caret at the beginning of the pattern ‘^PYTHON’ ensures that you match the word Python only at the beginning of the string. In the previous example, this doesn’t make any difference. But in the next example, it does:<\/p>\n
>>> re.findall('^PYTHON', 'PYTHON! PYTHON is fun')\n['PYTHON']<\/pre>\n
Although there are two occurrences of the substring ‘PYTHON’, there’s only one matching substring—at the beginning of the string.<\/p>\n
But what if you want to match not only at the beginning of the string but at the beginning of each line in a multi-line string? In other words:<\/p>\n
Python Re Start-of-Line (^) Regex<\/h3>\n
The caret operator, per default, only applies to the start of a string. So if you’ve got a multi-line string—for example, when reading a text file—it will still only match once: at the beginning of the string.<\/p>\n
However, you may want to match at the beginning of each line. For example, you may want to find all lines that start with ‘Python’ in a given string.<\/p>\n
You can specify that the caret operator matches the beginning of each line via the re.MULTILINE flag. Here’s an example showing both usages—without and with setting the re.MULTILINE flag:<\/p>\n
>>> import re\n>>> text = '''\nPython is great.\nPython is the fastest growing\nmajor programming language in\nthe world.\nPythonistas thrive.'''\n>>> re.findall('^Python', text)\n[]\n>>> re.findall('^Python', text, re.MULTILINE)\n['Python', 'Python', 'Python']\n>>> <\/pre>\n
The first output is the empty list because the string ‘Python’ does not appear at the beginning of the string. <\/p>\n
The second output is the list of three matching substrings because the string ‘Python’ appears three times at the beginning of a line.<\/p>\n
Python re.sub()<\/h3>\n
The re.sub(pattern, repl, string, count=0, flags=0)<\/strong> method returns a new string where all occurrences of the pattern in the old string are replaced by repl. Read more in the Finxter blog tutorial<\/a>.<\/p>\n
You can use the caret operator to substitute wherever some pattern appears at the beginning of the string:<\/p>\n
>>> import re\n>>> re.sub('^Python', 'Code', 'Python is \\nPython') 'Code is \\nPython'<\/pre>\n
Only the beginning of the string matches the regex pattern so you’ve got only one substitution.<\/p>\n
Again, you can use the re.MULTILINE flag to match the beginning of each line with the caret operator:<\/p>\n
>>> re.sub('^Python', 'Code', 'Python is \\nPython', flags=re.MULTILINE) 'Code is \\nCode'<\/pre>\n
Now, you replace both appearances of the string ‘Python’.<\/p>\n
Python re.match(), re.search(), re.findall(), and re.fullmatch()<\/h3>\n
Let’s quickly recap the most important regex methods in Python:<\/p>\n