Regular Expressions

re CS #

Look-arounds #

• Lookahead and lookbehind, collectively called “lookaround”
• Lookarounds are zero width assertions.
• They check for a regex (towards right or left of the current position - based on ahead or behind), succeeds or fails when a match is found (based on if it is positive or negative) and discards the matched portion.
• metaphor of ahead and behind is used wrt to left to right direction
• (?): are always present
  • !: means negative
  • =: means positive
  • <: look behind (direction)

Look-ahead #

• (?=): positive A(?=B): Find expression A where expression B follows:

- ~(?!): negative ~A(?!B): Find expression A where expression B does not follow:

Look-behind #

• (?<=): positive (?<=B)A: Find expression A where expression B precedes:

• (?<!): negative

(?<!B)A: Find expression A where expression B does not precede

Example #

bar(?=bar)     finds the 1st bar ("bar" which has "bar" after it)
bar(?!bar)     finds the 2nd bar ("bar" which does not have "bar" after it)
(?<=foo)bar    finds the 1st bar ("bar" which has "foo" before it)
(?<!foo)bar    finds the 2nd bar ("bar" which does not have "foo" before it)

Refs #

• lookaround
• lookaround examples
• good stackoverflow explanation

Image #

\w (word character) matches any single letter, number or underscore (same as [a-zA-Z0-9_])*.

Python suspendedfc #

ref

Match object #

• A Match Object is an object containing information about the search and the result.
• This object is returned by `re.search()`, `re.match()`, `re.findall()`.
• Has propterties and methods

  1. `.span()` returns a tuple containing the start-, and end positions of the match.

  2. `.string` returns the string passed into the function

  3. `.group()` returns the part of the string where there was a match

     import re
     txt = "My name is Javeed Ali Khan"
     result = re.search(r"(\bJ\w+) (\bA[li]{2})", txt)
     print(result.span())
     print(result.group()) # all match
     print(result.group(0)) # all match
     print(result.group(1)) # 1st group
     print(result.group(2)) # 2nd group
     print(result.string)
     

     (11, 21)
     Javeed Ali
     Javeed Ali
     Javeed
     Ali
     My name is Javeed Ali Khan
     

re methods 1 #

ref

re method Objects 1 #

re methods 2 #

Difference between match , search and findall #

ref

findall example

``` cc_list = ‘‘‘Ezra Koenig <ekoenig@vpwk.com>, Rostam Batmanglij <rostam@vpwk.com>, Chris Tomson <ctomson@vpwk. Bobbi Baio <bbaio@vpwk.com’’’ >>> matched = re.findall(r’\w+\@\w+\.\w+’, cc_list) >>> matched [’ekoenig@vpwk.com’, ‘rostam@vpwk.com’, ‘ctomson@vpwk.com’, ‘cbaio@vpwk.com’] >>> matched = re.findall(r’(\w+)\@(\w+)\.(\w+)’, cc_list) >>> matched [(’ekoenig’, ‘vpwk’, ‘com’), (‘rostam’, ‘vpwk’, ‘com’), (‘ctomson’, ‘vpwk’, ‘com’), (‘cbaio’, ‘vpwk’, ‘com’)] >>> names = [x[0] for x in matched] >>> names [’ekoenig’, ‘rostam’, ‘ctomson’, ‘cbaio’] ```

Examples #

Named group #

```(?P<name>regex)```

print("start")
import re
txt = """Javeed Ali Khan Mohammed 2284440597 lisai Taaina Immune by first dose Last update: Tuesday 8 June, 09:42 PM Immune by first dose until 22/10/2021 New Services Display All > COVID-19 Vaccine Certify Mobile Organ Health Donation Passport Number D00 000 of Javeed Ali Khan Mohammed 2284440597 lisai Taaina Immune by first dose """
txt = """Immune by first dose Last update Mon, 14 Jun 12:00 PM O Current Permits Immune by first dose Last update Mon, 14 Jun 12:00 PM O Current Permits"""

# pattern = r"""(?P<day_of_week>(mon|tues|wed(nes)?|thur(s)?|fri|sat(ur)?|sun)(day)?)(?<day_of_month>\s*\d+)(?<month>\s*(?:Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)?|May|Jun(?:e)?|Jul(?:y)?|Aug(?:ust)?|Sep(?:tember)?|Oct(?:ober)?|(Nov|Dec)(?:ember)?),\s*)(?p<time>1[0-2]|0?[1-9]):([0-5][0-9]) ?([AaPp][Mm])"""

pattern = r"""(?P<day_of_week>(mon|tues|wed(nes)?|thur(s)?|fri|sat(ur)?|sun)(day)?,?)(?P<day_of_month>\s*\d+)(?P<month>\s*(?:Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)?|May|Jun(?:e)?|Jul(?:y)?|Aug(?:ust)?|Sep(?:tember)?|Oct(?:ober)?|(Nov|Dec)(?:ember)?),?\s*)(?P<time>(1[0-2]|0?[1-9]):([0-5][0-9]) ?([AaPp][Mm]))"""
# pattern = r"""((mon|tues|wed(nes)?|thur(s)?|fri|sat(ur)?|sun)(day)?)(\s*\d+)(\s*(?:Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)?|May|Jun(?:e)?|Jul(?:y)?|Aug(?:ust)?|Sep(?:tember)?|Oct(?:ober)?|(Nov|Dec)(?:ember)?),\s*)(1[0-2]|0?[1-9]):([0-5][0-9]) ?([AaPp][Mm])"""

result = re.search(pattern, txt, re.IGNORECASE)
print("result is")
print(result)
if result:
    print(result.group("day_of_week"))
    print(result.group("day_of_month"))
    print(result.group("month"))
    print(result.group("time"))

start
result is
<re.Match object; span=(33, 53), match='Mon, 14 Jun 12:00 PM'>
Mon,
 14
 Jun
12:00 PM

Emacs #

Native regular expression support #

• Grouping is performed with backslashes followed by parentheses ‘\(’, ‘\)’
• " “: space not “\s\”
• look-ahead and look-behind are not supported
• supported constructs

“visual-regexp-steroids” package #

github depends on visual-regexp

Example #

to replace all “sample.html” to “sample” ([a-z0-9]*)\.html -> \1 # “\1” captures the group “([a-z0-9]*)”