What does this regex mean?
^[\w*]$
Quick answer: ^[\w*]$ will match a string consisting of a single character, where that character is alphanumeric (letters, numbers) an underscore (_) or an asterisk (*).
Details:
The "\w" means "any word character" which usually means alphanumeric (letters, numbers, regardless of case) plus underscore (_)
The "^" "anchors" to the beginning of a string, and the "$" "anchors" To the end of a string, which means that, in this case, the match must start at the beginning of a string and end at the end of the string.
The [] means a character class, which means "match any character contained in the character class".
It is also worth mentioning that normal quoting and escaping rules for strings make it very difficult to enter regular expressions (all the backslashes would need to be escaped with additional backslashes), so in Python there is a special notation which has its own special quoting rules that allow for all of the backslashes to be interpreted properly, and that is what the "r" at the beginning is for.
Note: Normally an asterisk (*) means "0 or more of the previous thing" but in the example above, it does not have that meaning, since the asterisk is inside of the character class, so it loses its "special-ness".
For more information on regular expressions in Python, the two official references are the re module, the Regular Expression HOWTO.
As exhuma said, \w is any word-class character (alphanumeric as Jonathan clarifies).
However because it is in square brackets it will match:
a single alphanumeric character OR
an asterisk (*)
So the whole regular expression matches:
the beginning of a
line (^)
followed by either a
single alphanumeric character or an
asterisk
followed by the end of a
line ($)
so the following would match:
blah
z <- matches this line
blah
or
blah
* <- matches this line
blah
\w refers to 0 or more alphanumeric characters and the underscore. the * in your case is also inside the character class, so [\w*] would match all of [a-zA-Z0-9_*] (the * is interpreted literally)
See http://www.regular-expressions.info/reference.html
To quote:
\d, \w and \s --- Shorthand character classes matching digits, word characters, and whitespace. Can be used inside and outside character classes.
Edit corrected in response to comment
From the beginning of this line, "Any number of word characters (letter, number, underscore)" until the end of the line.
I am unsure as to why it's in square brackets, as circle brackets (e.g. "(" and ")") are correct if you want the matched text returned.
\w is equivalent to [a-zA-Z0-9_] I don't understand the * after it or the [] around it, because \w already is a class and * in class definitions makes no sense.
As said above \w means any word. so you could use this in the context of below
view.aspx?url=[\w]
which means you can have any word as the value of the "url=" parameter
Related
I need a regular expression able to match everything but a string starting with a specific pattern (specifically index.php and what follows, like index.php?id=2342343).
Regex: match everything but:
a string starting with a specific pattern (e.g. any - empty, too - string not starting with foo):
Lookahead-based solution for NFAs:
^(?!foo).*$
^(?!foo)
Negated character class based solution for regex engines not supporting lookarounds:
^(([^f].{2}|.[^o].|.{2}[^o]).*|.{0,2})$
^([^f].{2}|.[^o].|.{2}[^o])|^.{0,2}$
a string ending with a specific pattern (say, no world. at the end):
Lookbehind-based solution:
(?<!world\.)$
^.*(?<!world\.)$
Lookahead solution:
^(?!.*world\.$).*
^(?!.*world\.$)
POSIX workaround:
^(.*([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.])|.{0,5})$
([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.]$|^.{0,5})$
a string containing specific text (say, not match a string having foo):
Lookaround-based solution:
^(?!.*foo)
^(?!.*foo).*$
POSIX workaround:
Use the online regex generator at www.formauri.es/personal/pgimeno/misc/non-match-regex
a string containing specific character (say, avoid matching a string having a | symbol):
^[^|]*$
a string equal to some string (say, not equal to foo):
Lookaround-based:
^(?!foo$)
^(?!foo$).*$
POSIX:
^(.{0,2}|.{4,}|[^f]..|.[^o].|..[^o])$
a sequence of characters:
PCRE (match any text but cat): /cat(*SKIP)(*FAIL)|[^c]*(?:c(?!at)[^c]*)*/i or /cat(*SKIP)(*FAIL)|(?:(?!cat).)+/is
Other engines allowing lookarounds: (cat)|[^c]*(?:c(?!at)[^c]*)* (or (?s)(cat)|(?:(?!cat).)*, or (cat)|[^c]+(?:c(?!at)[^c]*)*|(?:c(?!at)[^c]*)+[^c]*) and then check with language means: if Group 1 matched, it is not what we need, else, grab the match value if not empty
a certain single character or a set of characters:
Use a negated character class: [^a-z]+ (any char other than a lowercase ASCII letter)
Matching any char(s) but |: [^|]+
Demo note: the newline \n is used inside negated character classes in demos to avoid match overflow to the neighboring line(s). They are not necessary when testing individual strings.
Anchor note: In many languages, use \A to define the unambiguous start of string, and \z (in Python, it is \Z, in JavaScript, $ is OK) to define the very end of the string.
Dot note: In many flavors (but not POSIX, TRE, TCL), . matches any char but a newline char. Make sure you use a corresponding DOTALL modifier (/s in PCRE/Boost/.NET/Python/Java and /m in Ruby) for the . to match any char including a newline.
Backslash note: In languages where you have to declare patterns with C strings allowing escape sequences (like \n for a newline), you need to double the backslashes escaping special characters so that the engine could treat them as literal characters (e.g. in Java, world\. will be declared as "world\\.", or use a character class: "world[.]"). Use raw string literals (Python r'\bworld\b'), C# verbatim string literals #"world\.", or slashy strings/regex literal notations like /world\./.
You could use a negative lookahead from the start, e.g., ^(?!foo).*$ shouldn't match anything starting with foo.
You can put a ^ in the beginning of a character set to match anything but those characters.
[^=]*
will match everything but =
Just match /^index\.php/, and then reject whatever matches it.
In Python:
>>> import re
>>> p='^(?!index\.php\?[0-9]+).*$'
>>> s1='index.php?12345'
>>> re.match(p,s1)
>>> s2='index.html?12345'
>>> re.match(p,s2)
<_sre.SRE_Match object at 0xb7d65fa8>
Came across this thread after a long search. I had this problem for multiple searches and replace of some occurrences. But the pattern I used was matching till the end. Example below
import re
text = "start![image]xxx(xx.png) yyy xx![image]xxx(xxx.png) end"
replaced_text = re.sub(r'!\[image\](.*)\(.*\.png\)', '*', text)
print(replaced_text)
gave
start* end
Basically, the regex was matching from the first ![image] to the last .png, swallowing the middle yyy
Used the method posted above https://stackoverflow.com/a/17761124/429476 by Firish to break the match between the occurrence. Here the space is not matched; as the words are separated by space.
replaced_text = re.sub(r'!\[image\]([^ ]*)\([^ ]*\.png\)', '*', text)
and got what I wanted
start* yyy xx* end
I need a regular expression able to match everything but a string starting with a specific pattern (specifically index.php and what follows, like index.php?id=2342343).
Regex: match everything but:
a string starting with a specific pattern (e.g. any - empty, too - string not starting with foo):
Lookahead-based solution for NFAs:
^(?!foo).*$
^(?!foo)
Negated character class based solution for regex engines not supporting lookarounds:
^(([^f].{2}|.[^o].|.{2}[^o]).*|.{0,2})$
^([^f].{2}|.[^o].|.{2}[^o])|^.{0,2}$
a string ending with a specific pattern (say, no world. at the end):
Lookbehind-based solution:
(?<!world\.)$
^.*(?<!world\.)$
Lookahead solution:
^(?!.*world\.$).*
^(?!.*world\.$)
POSIX workaround:
^(.*([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.])|.{0,5})$
([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.]$|^.{0,5})$
a string containing specific text (say, not match a string having foo):
Lookaround-based solution:
^(?!.*foo)
^(?!.*foo).*$
POSIX workaround:
Use the online regex generator at www.formauri.es/personal/pgimeno/misc/non-match-regex
a string containing specific character (say, avoid matching a string having a | symbol):
^[^|]*$
a string equal to some string (say, not equal to foo):
Lookaround-based:
^(?!foo$)
^(?!foo$).*$
POSIX:
^(.{0,2}|.{4,}|[^f]..|.[^o].|..[^o])$
a sequence of characters:
PCRE (match any text but cat): /cat(*SKIP)(*FAIL)|[^c]*(?:c(?!at)[^c]*)*/i or /cat(*SKIP)(*FAIL)|(?:(?!cat).)+/is
Other engines allowing lookarounds: (cat)|[^c]*(?:c(?!at)[^c]*)* (or (?s)(cat)|(?:(?!cat).)*, or (cat)|[^c]+(?:c(?!at)[^c]*)*|(?:c(?!at)[^c]*)+[^c]*) and then check with language means: if Group 1 matched, it is not what we need, else, grab the match value if not empty
a certain single character or a set of characters:
Use a negated character class: [^a-z]+ (any char other than a lowercase ASCII letter)
Matching any char(s) but |: [^|]+
Demo note: the newline \n is used inside negated character classes in demos to avoid match overflow to the neighboring line(s). They are not necessary when testing individual strings.
Anchor note: In many languages, use \A to define the unambiguous start of string, and \z (in Python, it is \Z, in JavaScript, $ is OK) to define the very end of the string.
Dot note: In many flavors (but not POSIX, TRE, TCL), . matches any char but a newline char. Make sure you use a corresponding DOTALL modifier (/s in PCRE/Boost/.NET/Python/Java and /m in Ruby) for the . to match any char including a newline.
Backslash note: In languages where you have to declare patterns with C strings allowing escape sequences (like \n for a newline), you need to double the backslashes escaping special characters so that the engine could treat them as literal characters (e.g. in Java, world\. will be declared as "world\\.", or use a character class: "world[.]"). Use raw string literals (Python r'\bworld\b'), C# verbatim string literals #"world\.", or slashy strings/regex literal notations like /world\./.
You could use a negative lookahead from the start, e.g., ^(?!foo).*$ shouldn't match anything starting with foo.
You can put a ^ in the beginning of a character set to match anything but those characters.
[^=]*
will match everything but =
Just match /^index\.php/, and then reject whatever matches it.
In Python:
>>> import re
>>> p='^(?!index\.php\?[0-9]+).*$'
>>> s1='index.php?12345'
>>> re.match(p,s1)
>>> s2='index.html?12345'
>>> re.match(p,s2)
<_sre.SRE_Match object at 0xb7d65fa8>
Came across this thread after a long search. I had this problem for multiple searches and replace of some occurrences. But the pattern I used was matching till the end. Example below
import re
text = "start![image]xxx(xx.png) yyy xx![image]xxx(xxx.png) end"
replaced_text = re.sub(r'!\[image\](.*)\(.*\.png\)', '*', text)
print(replaced_text)
gave
start* end
Basically, the regex was matching from the first ![image] to the last .png, swallowing the middle yyy
Used the method posted above https://stackoverflow.com/a/17761124/429476 by Firish to break the match between the occurrence. Here the space is not matched; as the words are separated by space.
replaced_text = re.sub(r'!\[image\]([^ ]*)\([^ ]*\.png\)', '*', text)
and got what I wanted
start* yyy xx* end
I am new to python and i need to work on an existing python script. Can someone explain me what is the meaning of the following statement
pgre = re.compile("([^T]+)T([^\.]+)\.[^\s]+\s(\d+\.\d+):\s\[.+\]\s+(\d+)K->(\d+)K\((\d+)K\),\s(\d+\.\d+)\ssecs\]")
You need to consult the references for the exact meanings of each part of that regular expression, but the basic purpose of it is to parse the GC logging. Each parenthesized part of the expression () is a group that matches a useful part of the GC line.
For example, the start of the regex ([^T]+)T matches everything up to the first "T", and the grouped part returns the text before the "T", i.e. the date "2013-08-28"
The content of the group, [^T]+ means "at least one character that is not a T"
Patterns in square brackets [] are character classes - consult the references in the comments above for details. Note that your input text contains literal square brackets, so the pattern handles those with the \[ escape sequence - see below.
I think you can simplify ([^T]+)T to just (.+)T, incidentally.
Other useful sub-patterns:
\s matches whitespace
\d matches numeric digits
\. \( and \[ match literal periods, parentheses, and square braces, respectively, rather than interpreting them as special regex characters
I'm new to Stack so not sure if I'm asking this right.
I'm trying to form a regular expression to match all characters except 3 specific ones (%,&,and$) but I want to ignore that exception if a backslash () proceeds any of those characters. For example, if I have the string
abcd\$&
I would want the regular expression to match
abcd\$
because a backsplash preceeds the dollar sign, but not match the ^ because no backslash precedes it.
So far I have:
^[^%$&]+
which matches any string that doesn't have the characters (%, $, or &), but it stops at the backslash rather than include the backslash and the next character.
Thanks in advance!
^([^%$&\\]|\\.)+$
should work.
It also excludes \ from the charset and then allows \ followed by any character.
This is my attempt
def matcher(ex):
if re.match(r'^[\w|\d][A-Za-z0-9_-]+$', ex):
print 'yes'
My goal is to match only submission that satisfy all the followings
begins with only a letter or a numeric digit, and
only letter, space, dash, underscore and numeric digit are allowed
all ending spaces are stripped
In my regex, matcher('__') is considered valid. How can I modify to achieve what I want really want? I believe \w also includes underscore. But matcher('_') is not matched...
def matcher(ex):
ex = ex.rstrip()
if re.match(r'^[a-zA-Z0-9][ A-Za-z0-9_-]*$', ex):
print 'yes'
Problems in your original regex:
| doesn't mean alternation in a character class, it means a pipe character literally.
You used + for your following characters, meaning one or more, so a one-character string like '_' wouldn't match.
You used \w in your first character, which accepted underscores.