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Python - RegEx Library

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• Python - RegEx Library
  • RegEx
  • RegEx Functions
  • Raw String Notation: r"row_string" Prefix
  • Metacharacters
  • Special Sequences
  • Sets
  • Flags
  • Match Object

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RegEx

• RegEx / Regular Expression:

  • a set of characters with highly specialized syntax used to find or match other characters or groups of characters.

• re Module

  • Import
    • import re
  • Raise exception
    • re.error

• metacharacters

  • characters with a special meaning when utilized in a regular expression

• Greedy repetitions:

  • cause the matching algorithm to attempt to replicate the RE as many times as feasible. If later elements of the sequence fail to match, the matching algorithm will retry with lesser repetitions.

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RegEx Functions

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

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Raw String Notation: r"row_string" Prefix

• How to match a literal backslash and special escape sequences in Python string literals is a problem

• The solution is to use Python’s raw string notation for regular expressions, in which backslashes are not handled in any special way in a string literal prefixed with r.

• Example:

  Regular String	Raw string
  "ab\*"	r"ab\*"
  "\\\\section"	r"\\section"
  "\\w+\\s+\\1"	r"\w+\s+\1"

• The r prefix, making the literal a raw string literal.

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Metacharacters

• Metacharacters are characters with a special meaning.

Character	Description
^	Caret - Starts with	"^hello"
$	Dolla - Ends with	"planet$"
.	Dot - Any character (except newline character)	"he..o"
[]	Bracket - the set of characters	"[a-m]"
?	Question mark - Zero or one occurrences	"he.?o"
+	Plus - One or more occurrences	"he.+o"
*	Asterisk - Zero or more occurrences	"he.*o"
{}	Curly Braces - Exactly the specified number of occurrences	"he.{2}o"
\	Signals a special sequence (can also be used to escape special characters)	"\d"
`	`	Pipe - Either or "falls | stays"
()	Capture and group

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Special Sequences

Character	Matches
\d	any digit character == [0-9]	"\d"
\D	non-digit character == [^0-9]	"\D"
\s	any white space character	"\s"
\S	any character except the white space character	"\S"
\w	any alphanumeric character == [a-zA-Z0-9]	"\w"
\W	any characters except the alphanumeric character == [^a-zA-Z0-9]	"\W"
\A	the defined pattern at the start of the string	"\AThe"
\b	the pattern at the beginning or at the end of a word	r"\bain" / r"ain\b"
\B	the opposite of \b.	r"\Bain" r"ain\B"
\Z	the pattern is at the end of the string	"Spain\Z"

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Sets

• A set is a set of characters inside a pair of square brackets [] with a special meaning

Set	Match
[arn]	any of the specified characters (a, r, or n)
[a-n]	alphabetically between a and n
[^arn]	any character EXCEPT a, r, and n
[0123]	any of the specified digits (0, 1, 2, or 3) are present
[0-9]	any digit between 0 and 9
[0-5][0-9]	any two-digit numbers from 00 and 59
[a-zA-Z]	any character alphabetically between a and z, lower case OR upper case
[+]	any + character in the string

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Flags

Set	Match
re.A/ re.ASCII	ASCII-only matching
re.I/ re.IGNORECASE	case-insensitive matching
re.M/ re.MULTILINE	each newline matching

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Match Object

• A Match Object is an object containing information about the search and the result.

  • Note: If there is no match, the value None will be returned, instead of the Match Object.

• The Match object has properties and methods used to retrieve information about the search, and the result.

  • .span(): returns a tuple containing the start-, and end positions of the match.
  • .string: returns the string passed into the function,返回 search 方法的文本参数
  • .group(): returns the part of the string where there was a match.返回匹配字符

import re
print("\n--------Match Object.span()--------\n")


txt = "The rain in Spain"
xMatch = re.search(r"\bS\w+", txt)
print("span():\t\t", xMatch.span())         # span():          (12, 17)
print("start():\t", xMatch.start())         # start():         12
print("end():\t\t", xMatch.end())           # end():           17
print("group():\t", xMatch.group())         # group():         Spain

print("re:\t\t", xMatch.re)                 # re:              re.compile('\\bS\\w+')
print("string:\t\t", xMatch.string)         # string:          The rain in Spain

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