Regular Expression
Regular Expression¶
debugger https://regex101.com/r/qqbZqh/2
toturial https://regexone.com/problem/matching_html
Regular expressions are extremely useful in extracting information from text such as code, log files, spreadsheets, or even documents.
Metacharacters¶
abc… Letters
123… Digits
\d Any Digit
\D Any Non-digit character
. Any Character → (escape) \. Period
[abc] Only a, b, or c
[^abc] Not a, b, nor c
[a-z] Characters a to z
[0-9] Numbers 0 to 9
[^n-p] character except for letters n to p
\w Any Alphanumeric character
equivalent to the character range [A-Za-z0-9_]
\W Any Non-alphanumeric character
\b the boundary between a word and a non-word character.
- It's most useful in capturing entire words (for example by using the pattern \w+\b)
{m} m Repetitions
{m,n} m to n Repetitions
- a{3}
- [^1-9]{2,6}
- .{1,3}
* Zero or more repetitions
+ One or more repetitions
- .*
- aa+bc+
- (,\d+)
* and + are, by default, "greedy". They will match as much text as they can.
- ^.*:
ab:cde:f
? Optional character → (escape) \? Question mark
- ab?c : match either the strings "abc" or "ac" because the b is considered optional
\s Any Whitespace
The most common forms of whitespace you will use with regular expressions are the space (
␣), the tab (\t), the new line (\n) and the carriage return (\r) (useful in Windows environments), and these special characters match each of their respective whitespaces. In addition, a whitespace special character\swill match any of the specific whitespaces above and is extremely useful when dealing with raw input text.\SAny Non-whitespace character
It is often best practice to write as specific regular expressions as possible to ensure that we don't get false positives when matching against real world text.
e.g. We wanted to match the word "success" in a log file. We certainly don't want that pattern to match a line that says "Error: unsuccessful operation"!
^…$ Starts and ends
- ^succuss
Note that this is different than the hat used inside a set of bracket [^...] for excluding characters
Regular expressions allow us to not just match text but also to extract information for further processing.
(…) Capture Group
- ^(IMG\d+.png)$ captures and extracts the full filename
- ^(IMG\d+).png$ captures the part before the period.
(a(bc)) Capture Sub-group
- ^(IMG(\d+)).png$
Generally, the results of the captured groups are in the order in which they are defined (in order by open parenthesis).
(.*) Capture all
Specifically when using groups, you can use the | (logical OR, aka. the pipe) to denote different possible sets of characters.
(abc|def) Matches abc or def
back referencing Many systems allow you to reference your captured groups by using \0 (usually the full matched text), \1 (group 1), \2 (group 2), etc.
Examples¶
Matching decimal numbers
3.14529
-255.34
128
1.9e10
123,340.00
Task Text Capture Groups
capture 415-555-1234 415
capture 650-555-2345 650
capture (416)555-3456 416
capture 202 555 4567 202
capture 4035555678 403
capture 1 416 555 9292 416
Matching emails
Task Text Capture Groups
capture tom@hogwarts.com tom
capture tom.riddle@hogwarts.com tom.riddle
capture tom.riddle+regexone@hogwarts.com tom.riddle
capture tom@hogwarts.eu.com tom
capture potter@hogwarts.com potter
capture harry@hogwarts.com harry
capture hermione+regexone@hogwarts.com hermione