8.10. Syntax Extension

  • In other programming languages

  • PCRE - Perl Compatible Regular Expressions

../../_images/regex-xkcd-standards.png

Figure 8.2. How Standards Proliferate. XKCD Standards 1

8.10.1. Future

8.10.2. Named Ranges

  • [:allnum:] - Alphabetic and numeric character [a-zA-Z0-9]

  • [:alpha:] - Alphabetic character [a-zA-Z]

  • [:alnum:] - Alphabetic and numeric character [a-zA-Z0-9]

  • [:alpha:] - Alphabetic character [a-zA-Z]

  • [:blank:] - Space or tab

  • [:cntrl:] - Control character

  • [:digit:] - Digit

  • [:graph:] - Non-blank character (excludes spaces, control characters, and similar)

  • [:lower:] - Lowercase alphabetical character

  • [:print:] - Like [:graph:], but includes the space character

  • [:punct:] - Punctuation character

  • [:space:] - Whitespace character ([:blank:], newline, carriage return, etc.)

  • [:upper:] - Uppercase alphabetical

  • [:xdigit:] - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)

  • [:word:] - A character in one of the following Unicode general categories Letter, Mark, Number, Connector_Punctuation

  • [:ascii:] - A character in the ASCII character set

In Python those Named Ranges does not work. String [:alpha:] will be interpreted literally as either: : or a or l or p or h or a.

>>> import re
>>> TEXT = 'hello world'
>>>
>>>
>>> re.findall('[:allnum:]', TEXT)
['l', 'l', 'l']
>>>
>>> re.findall('[:alpha:]', TEXT)
['h', 'l', 'l', 'l']

8.10.3. Range

  • [a-Z] == [a-zA-Z]

  • [a-9] == [a-zA-Z0-9]

  • Works in other languages, but not in Python

>>> import re
>>> TEXT = 'hello world'
>>>
>>>
>>> re.findall('[a-Z]', TEXT)
Traceback (most recent call last):
re.error: bad character range a-Z at position 1
>>>
>>> re.findall('[a-9]', TEXT)
Traceback (most recent call last):
re.error: bad character range a-9 at position 1

8.10.4. Group Backreference

  • $1 == \1

>>> HTML = '<span>Hello World</span>'
>>> re.findall('<(?P<tag>.+)>(?:.+)</(?P=tag)>', HTML)
['span']

8.10.5. References

1

Munroe, R. How Standards Proliferate. Year: 2022. Retrieved: 2022-04-27. URL: https://xkcd.com/927/