Regular Expressions (in Python)
[Pages:26]Regular Expressions (in Python)
Python or Egrep
? We will use Python. ? In some scripting languages you can call the
command "grep" or "egrep" ? egrep pattern file.txt ? E.g. egrep "^A" file.txt ? Will print all the line of file.txt which start with
(^) the letter A (capital A)
Regular expression
? (abbreviated regex or regexp) a search pattern, mainly for use in pattern matching with strings, i.e. "find and replace"like operations.
? Each character in a regular expression is either understood to be a metacharacter with its special meaning, or a regular character with its literal meaning.
? We ask the question ? does a given string match a certain pattern?
List of Meta characters
1. . 2. + 3. ? 4. * 5. ^ 6. $ 7. [...] 8. 9. [^...] 10. | 11. () 12. {m,n}
. (dot)
? Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, characterencoding-, and platform-specific, but it is safe to assume that the line feed character is included).
? Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
Example .
? string1 = "Hello, world."
? if re.search(r".....", string1):
?
print string1 + " has length
>= 5"
Example [.] literally a dot
? string1 = "Hello, world."
? if re.search(r"....[.]", string1):
?
print string1 + " has length
>= 5 and ends with a ."
+
? Matches the preceding element one or more times. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".
? string1 = "Hello, world."
? if re.search(r"l+", string1):
?
print 'There are one or more
consecutive letter "l"' +\
?
"'s in " + string1
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