Regular Expression
A regular expression is a sequence of characters that can be used as a search pattern. Sometimes referred to as Regex or Regexen.
One use of regular expressions is in search engines.
Special Metacharacters
A complete list of Python regular expression characters can be found here.
Often used special metacharacters include:
. matches any character except newline
^ matches the start of the string
$ matches the end of the string or just before a newline
* matches 0 or more repetitions of the preceding Regex
+ matches 1 or more repetitions of the preceding Regex
- indicates a range, an example is 1-5
{} repetitions of the previous elements; examples are {3} {2,4} {,4}
[] set of any characters to match; examples are [ab4] [a-z]
() capture group based on a Regex; an example is (a*)
| either or
\ escape to match special characters; an example is \+
\n newline
\d digit
\s space
Python Example
Python regular expression implementation details can be found here.
# Import the regular expression module. import re # Define text against which a regular expression will be used. text_sample = "This is text to test regular expressions." # Define a regular expression. regular_expression = "^This" # Search the text to see if it begins with specific characters. result = re.findall(regular_expression, text_sample) print(result)