4.10. Regex Syntax Flavors

  • In other programming languages

  • PCRE - Perl Compatible Regular Expressions

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

Figure 4.2. How Standards Proliferate. XKCD Standards [1]

4.10.1. SetUp

>>> import re

4.10.2. Future

4.10.3. Enclosing

  • In Python we use raw-string (r'...')

  • In JavaScript we use /pattern/flags or new RegExp(pattern, flags)

Python:

>>> string = 'hello'
>>> string = "hello"
>>> regex = r'[a-z]+'
>>> regex = r"[a-z]+"

JavaScript:

string = "hello"
regex = /[a-z]+/
regex = new RegExp('[a-z]+)

4.10.4. Flags

  • In Python we use raw-string (r'...')

  • In JavaScript we use /pattern/flags or new RegExp(pattern, flags)

Python:

>>> 
... re.findall(r'[a-z]+', TEXT, flags=re.I)
... re.findall(r'[a-z]+', TEXT, flags=re.IGNORECASE)

/[a-z]+/i
new RegExp("[a-z]", 'i')

4.10.5. 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(r'[:allnum:]', TEXT)
['l', 'l', 'l']
>>>
>>> re.findall(r'[:alpha:]', TEXT)
['h', 'l', 'l', 'l']

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

4.10.7. Group Backreference

  • $1 - grep, egrep, Jetbrains IDE

  • \1

  • \g<1> - Python

  • \g<name> - Python

In JavaScript name groups don't have ?P but only ?:

'(?P<name>\d+)'

'(?<name>\d+)'

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

>>> ARES = 'Mark Watney of Ares 3 landed on Mars on: Nov 7th, 2035 at 13:37'
>>>
>>> year = r'(?P<year>\d{4})'
>>> month = r'(?P<month>[A-Z][a-z]+)'
>>> day = r'(?P<day>\d{1,2})'
>>> date = f'{month} {day}(?:st|nd|rd|th), {year}'
>>>
>>> re.search(date, ARES).groupdict()
{'month': 'Nov', 'day': '7', 'year': '2035'}
>>>
>>> re.sub(date, '\g<year> \g<month> \g<day>', ARES)
'Mark Watney of Ares 3 landed on Mars on: 2035 Nov 7 at 13:37'
>>>
>>> re.sub(date, '\g<3> \g<1> \g<2>', ARES)
'Mark Watney of Ares 3 landed on Mars on: 2035 Nov 7 at 13:37'

4.10.8. References