# Regular expressions

TODO: ADD MORE

- the `re.sub()` function of the `re` module that replaces a `regular expression` match with a new value. The example solutions use this function in various places to substitute _markdown_ syntax for _HTML_ syntax in the passed in markdown text. [markdown](../exercise-concepts/markdown.md)
- Both the original code to be refactored for this exercise and the example solution import and use the `re` module for Regular Expressions in python. [markdown](../exercise-concepts/markdown.md)
- the `re.match()` function from the `re` module returns a `match` object with any matched values from a specified Regular Expression or pre-compiled Regular Expression. The example uses `re.match()` in multiple places to search for text patterns that need re-formatting or substituting. [markdown](../exercise-concepts/markdown.md)
- Various functions in the re module return a `re.Match` _instance_ which in turn has a `Match.group` method. `Match.group` exists even if there are no groups specified in the pattern. See the [Match.group docs](https://docs.python.org/3/library/re.html#re.Match.group) for more detail. [markdown](../exercise-concepts/markdown.md)
- regular expressions is a language of sorts that can detect substrings and extract groups from a string, as well as replace them with something else [phone-number](../exercise-concepts/phone-number.md)
- A Domain Specific Language (DSL) for text processing. Like many other programming languages in use, python supports a quasi-dialect of PCRE (_Perl compatible regular expressions_). `Regular expressions` can be used via the core python `re` module, or the third-party `regex` module. Both the original code to be refactored for this exercise and the example solutions use the core `re` module to access `regular expressions` functionality. [markdown](../exercise-concepts/markdown.md)