1. meaning of re keywords are very unusual than their regular meaning.
2. in general, matching text= match.group()
3. Regular expression: deals with text search in a string
\w = any word character match
+ = unlimited repeat of the preceding pattern-type (such as . \w \d ...) if one instance is found .
* = unlimited repeat of the preceding pattern-type (such as . \w \d ...) even if no instance is found .
https://docs.python.org/2/library/re.html
https://docs.python.org/2/howto/regex.html#regex-howto
https://developers.google.com/edu/python/regular-expressions?hl=en
Regular expressions use the backslash character ('\') to indicate special forms or to allow special characters to be used without invoking their special meaning.
The basic rules of regular expression search for a pattern within a string are:
- The search proceeds through the string from start to end, stopping at the first match found
- All of the pattern must be matched, but not all of the string
- If
match = re.search(pat, str)is successful, match is not None and in particular match.group() is the matching text
#Square bracket: => group of characters to match: [ ]: so [abc] matches 'a' or 'b' or 'c
#Group extraction: add parenthesis ( )
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Python 2.7 Regular Expressions
Non-special chars match themselves. Exceptions are special characters:
\ Escape special char or start a sequence. . Match any char except newline, see re.DOTALL ^ Match start of the string, see re.MULTILINE $ Match end of the string, see re.MULTILINE [] Enclose a set of matchable chars R|S Match either regex R or regex S. () Create capture group, & indicate precedence
After '
[', enclose a set, the only special chars are:] End the set, if not the 1st char - A range, eg. a-c matches a, b or c ^ Negate the set only if it is the 1st char
Quantifiers (append '
?' for non-greedy):{m} Exactly m repetitions
{m,n} From m (default 0) to n (default infinity)
* 0 or more. Same as {,}
+ 1 or more. Same as {1,}
? 0 or 1. Same as {,1}
Special sequences:
\A Start of string
\b Match empty string at word (\w+) boundary
\B Match empty string not at word boundary
\d Digit
\D Non-digit
\s Whitespace [ \t\n\r\f\v], see LOCALE,UNICODE
\S Non-whitespace
\w Alphanumeric: [0-9a-zA-Z_], see LOCALE
\W Non-alphanumeric
\Z End of string
\g<id> Match prev named or numbered group,
'<' & '>' are literal, e.g. \g<0>
or \g<name> (not \g0 or \gname)
Special character escapes are much like those already escaped in Python string literals. Hence regex '
\n' is same as regex '\\n':\a ASCII Bell (BEL)
\f ASCII Formfeed
\n ASCII Linefeed
\r ASCII Carriage return
\t ASCII Tab
\v ASCII Vertical tab
\\ A single backslash
\xHH Two digit hexadecimal character goes here
\OOO Three digit octal char (or just use an
initial zero, e.g. \0, \09)
\DD Decimal number 1 to 99, match
previous numbered group
Extensions. Do not cause grouping, except '
P<name>':(?iLmsux) Match empty string, sets re.X flags (?:...) Non-capturing version of regular parens (?P<name>...) Create a named capturing group. (?P=name) Match whatever matched prev named group (?#...) A comment; ignored. (?=...) Lookahead assertion, match without consuming (?!...) Negative lookahead assertion (?<=...) Lookbehind assertion, match if preceded (?<!...) Negative lookbehind assertion (?(id)y|n) Match 'y' if group 'id' matched, else 'n'
Flags for re.compile(), etc. Combine with
'|':re.I == re.IGNORECASE Ignore case
re.L == re.LOCALE Make \w, \b, and \s locale dependent
re.M == re.MULTILINE Multiline
re.S == re.DOTALL Dot matches all (including newline)
re.U == re.UNICODE Make \w, \b, \d, and \s unicode dependent
re.X == re.VERBOSE Verbose (unescaped whitespace in pattern
is ignored, and '#' marks comment lines)
Module level functions:
compile(pattern[, flags]) -> RegexObject match(pattern, string[, flags]) -> MatchObject search(pattner, string[, flags]) -> MatchObject findall(pattern, string[, flags]) -> list of strings finditer(pattern, string[, flags]) -> iter of MatchObjects split(pattern, string[, maxsplit, flags]) -> list of strings sub(pattern, repl, string[, count, flags]) -> string subn(pattern, repl, string[, count, flags]) -> (string, int) escape(string) -> string purge() # the re cache
RegexObjects (returned from
compile()):.match(string[, pos, endpos]) -> MatchObject
.search(string[, pos, endpos]) -> MatchObject
.findall(string[, pos, endpos]) -> list of strings
.finditer(string[, pos, endpos]) -> iter of MatchObjects
.split(string[, maxsplit]) -> list of strings
.sub(repl, string[, count]) -> string
.subn(repl, string[, count]) -> (string, int)
.flags # int, Passed to compile()
.groups # int, Number of capturing groups
.groupindex # {}, Maps group names to ints
.pattern # string, Passed to compile()
MatchObjects (returned from
match() and search()):.expand(template) -> string, Backslash & group expansion
.group([group1...]) -> string or tuple of strings, 1 per arg
.groups([default]) -> tuple of all groups, non-matching=default
.groupdict([default]) -> {}, Named groups, non-matching=default
.start([group]) -> int, Start/end of substring match by group
.end([group]) -> int, Group defaults to 0, the whole match
.span([group]) -> tuple (match.start(group), match.end(group))
.pos int, Passed to search() or match()
.endpos int, "
.lastindex int, Index of last matched capturing group
.lastgroup string, Name of last matched capturing group
.re regex, As passed to search() or match()
.string string, "
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Regular expression in programming: Elaborate feature on: Find a substring in a given string
1. It is related with finding something in a string.
2. Modern regular expression has heavy extended capabilities beyond find a substring.
3. There are specific mechanism to use regular expression capabilities of different langulages and operating systems.
Regular expression (definition): an expression to define criteria to find match
The special characters are for regular expression (RE):
'.' (Dot.) In the default mode, this matches any character except a newline. If the DOTALL flag has been specified, this matches any character including a newline.
'^' (Caret.) Matches the start of the string, and in MULTILINE mode also matches immediately after each newline.
'$' Matches the end of the string or just before the newline at the end of the string, and in MULTILINE mode also matches before a newline. foo matches both ‘foo’ and ‘foobar’, while the regular expression foo$ matches only ‘foo’. More interestingly, searching for foo.$ in 'foo1\nfoo2\n' matches ‘foo2’ normally, but ‘foo1’ in MULTILINE mode; searching for a single $ in 'foo\n' will find two (empty) matches: one just before the newline, and one at the end of the string.
'*' Causes the resulting RE to match 0 or more repetitions of the preceding RE, as many repetitions as are possible. ab* will match ‘a’, ‘ab’, or ‘a’ followed by any number of ‘b’s.
'+' Causes the resulting RE to match 1 or more repetitions of the preceding RE. ab+ will match ‘a’ followed by any non-zero number of ‘b’s; it will not match just ‘a’.
'?' Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. ab? will match either ‘a’ or ‘ab’.
*?, +?, ??
The '*', '+', and '?' qualifiers are all greedy; they match as much text as possible. Sometimes this behaviour isn’t desired; if the RE <.*> is matched against '<H1>title</H1>', it will match the entire string, and not just '<H1>'. Adding '?' after the qualifier makes it perform the match in non-greedy or minimal fashion; as few characters as possible will be matched. Using .*? in the previous expression will match only '<H1>'.
{m}
Specifies that exactly m copies of the previous RE should be matched; fewer matches cause the entire RE not to match. For example, a{6} will match exactly six 'a' characters, but not five.
{m,n}
Causes the resulting RE to match from m to n repetitions of the preceding RE, attempting to match as many repetitions as possible. For example, a{3,5} will match from 3 to 5 'a' characters. Omitting m specifies a lower bound of zero, and omitting n specifies an infinite upper bound. As an example, a{4,}b will match aaaab or a thousand 'a' characters followed by a b, but not aaab. The comma may not be omitted or the modifier would be confused with the previously described form.
{m,n}?
Causes the resulting RE to match from m to n repetitions of the preceding RE, attempting to match as few repetitions as possible. This is the non-greedy version of the previous qualifier. For example, on the 6-character string 'aaaaaa', a{3,5} will match 5 'a' characters, while a{3,5}? will only match 3 characters.
'\'
Either escapes special characters (permitting you to match characters like '*', '?', and so forth), or signals a special sequence; special sequences are discussed below.
If you’re not using a raw string to express the pattern, remember that Python also uses the backslash as an escape sequence in string literals; if the escape sequence isn’t recognized by Python’s parser, the backslash and subsequent character are included in the resulting string. However, if Python would recognize the resulting sequence, the backslash should be repeated twice. This is complicated and hard to understand, so it’s highly recommended that you use raw strings for all but the simplest expressions.
[]
Used to indicate a set of characters. In a set:
Characters can be listed individually, e.g. [amk] will match 'a', 'm', or 'k'.
Ranges of characters can be indicated by giving two characters and separating them by a '-', for example [a-z] will match any lowercase ASCII letter, [0-5][0-9] will match all the two-digits numbers from 00 to 59, and [0-9A-Fa-f] will match any hexadecimal digit. If - is escaped (e.g. [a\-z]) or if it’s placed as the first or last character (e.g. [a-]), it will match a literal '-'.
Special characters lose their special meaning inside sets. For example, [(+*)] will match any of the literal characters '(', '+', '*', or ')'.
Character classes such as \w or \S (defined below) are also accepted inside a set, although the characters they match depends on whether LOCALE or UNICODE mode is in force.
Characters that are not within a range can be matched by complementing the set. If the first character of the set is '^', all the characters that are not in the set will be matched. For example, [^5] will match any character except '5', and [^^] will match any character except '^'. ^ has no special meaning if it’s not the first character in the set.
To match a literal ']' inside a set, precede it with a backslash, or place it at the beginning of the set. For example, both [()[\]{}] and []()[{}] will both match a parenthesis.
'|'
A|B, where A and B can be arbitrary REs, creates a regular expression that will match either A or B. An arbitrary number of REs can be separated by the '|' in this way. This can be used inside groups (see below) as well. As the target string is scanned, REs separated by '|' are tried from left to right. When one pattern completely matches, that branch is accepted. This means that once A matches, B will not be tested further, even if it would produce a longer overall match. In other words, the '|' operator is never greedy. To match a literal '|', use \|, or enclose it inside a character class, as in [|].
(...)
Matches whatever regular expression is inside the parentheses, and indicates the start and end of a group; the contents of a group can be retrieved after a match has been performed, and can be matched later in the string with the \number special sequence, described below. To match the literals '(' or ')', use \( or \), or enclose them inside a character class: [(] [)].
(?...)
This is an extension notation (a '?' following a '(' is not meaningful otherwise). The first character after the '?' determines what the meaning and further syntax of the construct is. Extensions usually do not create a new group; (?P<name>...) is the only exception to this rule. Following are the currently supported extensions.
(?iLmsux)
(One or more letters from the set 'i', 'L', 'm', 's', 'u', 'x'.) The group matches the empty string; the letters set the corresponding flags: re.I (ignore case), re.L (locale dependent), re.M (multi-line), re.S (dot matches all), re.U (Unicode dependent), and re.X (verbose), for the entire regular expression. (The flags are described in Module Contents.) This is useful if you wish to include the flags as part of the regular expression, instead of passing a flag argument to the re.compile() function.
Note that the (?x) flag changes how the expression is parsed. It should be used first in the expression string, or after one or more whitespace characters. If there are non-whitespace characters before the flag, the results are undefined.
(?:...)
A non-capturing version of regular parentheses. Matches whatever regular expression is inside the parentheses, but the substring matched by the group cannot be retrieved after performing a match or referenced later in the pattern.
(?P<name>...)
Similar to regular parentheses, but the substring matched by the group is accessible via the symbolic group name name. Group names must be valid Python identifiers, and each group name must be defined only once within a regular expression. A symbolic group is also a numbered group, just as if the group were not named.
Named groups can be referenced in three contexts. If the pattern is (?P<quote>['"]).*?(?P=quote) (i.e. matching a string quoted with either single or double quotes):
Context of reference to group “quote” Ways to reference it
in the same pattern itself
(?P=quote) (as shown)
\1
when processing match object m
m.group('quote')
m.end('quote') (etc.)
in a string passed to the repl argument of re.sub()
\g<quote>
\g<1>
\1
(?P=name)
A backreference to a named group; it matches whatever text was matched by the earlier group named name.
(?#...)
A comment; the contents of the parentheses are simply ignored.
(?=...)
Matches if ... matches next, but doesn’t consume any of the string. This is called a lookahead assertion. For example, Isaac (?=Asimov) will match 'Isaac ' only if it’s followed by 'Asimov'.
(?!...)
Matches if ... doesn’t match next. This is a negative lookahead assertion. For example, Isaac (?!Asimov) will match 'Isaac ' only if it’s not followed by 'Asimov'.
(?<=...)
Matches if the current position in the string is preceded by a match for ... that ends at the current position. This is called a positive lookbehind assertion. (?<=abc)def will find a match in abcdef, since the lookbehind will back up 3 characters and check if the contained pattern matches. The contained pattern must only match strings of some fixed length, meaning that abc or a|b are allowed, but a* and a{3,4} are not. Group references are not supported even if they match strings of some fixed length. Note that patterns which start with positive lookbehind assertions will not match at the beginning of the string being searched; you will most likely want to use the search() function rather than the match() function:
Features in regular expression libraries:
1.
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