7.1. Regex About

  • Also known as: re, regex, regexp, Regular Expressions

W3C HTML5 Standard [4] regexp for email field:

>>> pattern = r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$"

7.1.1. Python

  • import re

  • re.findall() - find all occurrences of pattern in string, returns list[str]

  • re.finditer() - find first occurrence of pattern in string, returns Iterator[re.Match]

  • re.search() - find first occurrence of pattern in string, returns re.Match (stops after first match)

  • re.match() - check if string matches pattern, used in validation: phone, email, tax id, etc., returns re.Match

  • re.compile() - compile pattern into object for further use, for example in the loop, returns re.Pattern

  • re.split() - split string by pattern, returns list[str]

  • re.sub() - substitute pattern in string with something else, returns str

7.1.2. Syntax

  • Character Class - what to find (single character)

  • Qualifiers - range to find (range)

  • Negation

  • Quantifiers - how many occurrences of preceding qualifier or character class

  • Groups

  • Look Ahead and Look Behind

  • Flags

  • Extensions

  • [] - Qualifier

  • {} - Quantifier

  • () - Groups

7.1.3. Under the Hood

  • ASCII Table

  • chr() - returns a string representing a character whose Unicode code point is the integer passed, for example chr(65) returns 'A'

  • ord() - returns an integer representing the Unicode code point of the character, for example ord('A') returns 65

  • re.DEBUG - flag to show how regular expression is parsed and executed

>>> ord('A')
65

>>> string = 'Hello Alice'
>>>
>>> [ord(x) for x in string]
[72, 101, 108, 108, 111, 32, 65, 108, 105, 99, 101]

>>> import re
>>>
>>>
>>> string = 'Hello Alice'
>>>
>>> re.findall(r'A', string, flags=re.DEBUG)
LITERAL 65

 0. INFO 8 0b11 1 1 (to 9)
      prefix_skip 1
      prefix [0x41] ('A')
      overlap [0]
 9: LITERAL 0x41 ('A')
11. SUCCESS
['A']

7.1.4. Visualization

../../_images/regexp-visualization.png

Figure 7.4. Visualization for pattern r'^[a-zA-Z0-9][\w.+-]*@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]{2,20}$' [1]

7.1.5. Further Reading

7.1.6. References