Do You Need to Escape a Dot in a Python Regex Character Class? – Finxter

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Python Regex Dot


Question

Recap the following regular expression concepts (more details in the article below):

  • The dot character . in regular expressions matches any character excluding the newline character. For example, the pattern 'c.t' will match the strings 'cat', 'cut', or 'czt'.
  • The character class [ ] is a set of characters: if you use it in a regex pattern, the engine will match exactly one character from the set. For example, the pattern 'c[auz]t' will match the strings 'cat', 'cut', or 'czt'.
  • Special characters such as the dot character often need to be escaped in a regex pattern if you want to match them. For example, to match the actual dot '.' character, you need to design a pattern with escaped dot '.'. In other words, the pattern 'hello.' would match the string 'hello.' but not the string 'hello!'.

Question:

Do you need to escape the dot character in a Python regex character class?

Answer

No, you don’t need to escape the dot character in a character class. This holds for the Python re and the newer Python regex modules.

The reason is that in a character class, any character except ^, -, ] or are literals, i.e., they’ve lost their special meaning if they had any.

The Minus Character

For example, the minus '-' character has a special meaning within the character class, it’s the range character in the pattern '[a-z]'.

However, the minus is also seen as a normal literal character if it’s the first or last value in a character class. Python knows that the minus as the first or last character cannot signal a range because the range wouldn’t be opened or closed (e.g., patterns [-z] or [a-]).

The Hat Character

The hat special character '^' means start-of-the-line regex. It has another special meaning when used as the first character of the character class (=negative character class).

However, it loses its special meaning when it’s not the first character. So if you want to match the '^' symbol, you can use it as the non-first character in a character class (e.g., pattern [ab^c]).

💡 Note: It doesn’t harm to escape the dot regex or any other special symbol within the character class—Python will simply ignore it!

Let’s recap some of the basic concepts in more detail next!

Understanding the Dot Regex

The dot regex . matches all characters except the newline character.

For example, the regular expression '...' matches strings 'hey' and 'tom'. But it does not match the string 'yonto' which contains the newline character 'n'. Combined with the asterisk quantifier in the pattern '.*', the dot regex matches an arbitrary number of symbols except newline characters.

Python Regex Dot

Learn more in our detailed blog tutorial guide:

Understanding Character Classes

The character set (or character class) is, surprise, a set of characters: if you use a character set in a regular expression pattern, you tell the regex engine to choose one arbitrary character from the set. As you may know, a set is an unordered collection of unique elements. So each character in a character set is unique and the order doesn’t really matter (with a few minor exceptions).

Here’s an example of a character set as used in a regular expression:

>>> import re
>>> re.findall('[abcde]', 'hello world!')
['e', 'd']

Learn more in our detailed blog tutorial guide:

Understanding Special Characters

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