I am looking for a version of gsub which doesn't try to interpret its input as regular expressions and uses normal C-like escaped strings.
Update
The question was initiated by a strange behavior:
text.gsub("pattern", "\\\\\\")
and
text.gsub("pattern", "\\\\\\\\")
are treated as the same, and
text.gsub("pattern", "\\\\")
is treated as single backslash.
There are two layers of escaping for the second parameter of gsub:
The first layer is Ruby string constant. If it is written like \\\\\\ it is unescaped by Ruby like \\\
the second layer is gsub itself: \\\ is treated like \\ + \
double backslash is resolved into single: \\ => \ and the single trailing backslash at the end is resolved as itself.
8 backslashes are parsed in the similar way:
"\\\\\\\\" => "\\\\"
and then
"\\\\" => "\\"
so the constants consisting of six and eight backslashes are resolved into two backslashes.
To make life a bit easier, a block may be used in gsub function. String constants in a block are passed only through Ruby layer (thanks to #Sorrow).
"foo\\bar".gsub("\\") {"\\\\"}
gsub accepts strings as first parameter:
the pattern is typically a Regexp; if given as
a String, any regular expression metacharacters
it contains will be interpreted literally
Example:
"hello world, i am thy code".gsub("o", "-foo-")
=> "hell-foo- w-foo-rld, i am thy c-foo-de"
Related
I have a name spaced class..
"CommonCar::RedTrunk"
I need to convert it to an underscored string "common_car_red_trunk", but when I use
"CommonCar::RedTrunk".underscore, I get "common_car/red_trunk" instead.
Is there another method to accomplish what I need?
Solutions:
"CommonCar::RedTrunk".gsub(':', '').underscore
or:
"CommonCar::RedTrunk".sub('::', '').underscore
or:
"CommonCar::RedTrunk".tr(':', '').underscore
Alternate:
Or turn any of these around and do the underscore() first, followed by whatever method you want to use to replace "/" with "_".
Explanation:
While all of these methods look basically the same, there are subtle differences that can be very impactful.
In short:
gsub() – uses a regex to do pattern matching, therefore, it's finding any occurrence of ":" and replacing it with "".
sub() – uses a regex to do pattern matching, similarly to gsub(), with the exception that it's only finding the first occurrence (the "g" in gsub() meaning "global"). This is why when using that method, it was necessary to use "::", otherwise a single ":" would have been left. Keep in mind with this method, it will only work with a single-nested namespace. Meaning "CommonCar::RedTrunk::BigWheels" would have been transformed to "CommonCarRedTrunk::BigWheels".
tr() – uses the string parameters as arrays of single character replacments. In this case, because we're only replacing a single character, it'll work identically to gsub(). However, if you wanted to replace "on" with "EX", for example, gsub("on", "EX") would produce "CommEXCar::RedTrunk" while tr("on", "EX") would produce "CEmmEXCar::RedTruXk".
Docs:
https://apidock.com/ruby/String/gsub
https://apidock.com/ruby/String/sub
https://apidock.com/ruby/String/tr
This is a pure-Ruby solution.
r = /(?<=[a-z])(?=[A-Z])|::/
"CommonCar::RedTrunk".gsub(r, '_').downcase
#=> "common_car_red_trunk"
See (the first form of) String#gsub and String#downcase.
The regular expression can be made self-documenting by writing it in free-spacing mode:
r = /
(?<=[a-z]) # assert that the previous character is lower-case
(?=[A-Z]) # assert that the following character is upper-case
| # or
:: # match '::'
/x # free-spacing regex definition mode
(?<=[a-z]) is a positive lookbehind; (?=[A-Z]) is a positive lookahead.
Note that /(?<=[a-z])(?=[A-Z])/ matches an empty ("zero-width") string. r matches, for example, the empty string between 'Common' and 'Car', because it is preceeded by a lower-case letter and followed by an upper-case letter.
I don't know Rails but I'm guessing you could write
"CommonCar::RedTrunk".delete(':').underscore
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 want to fetch records that have some string field start with a given prefix and end on any one character. Basically:
Model.where('field LIKE ?', "#{prefix}_").count
The problem is that the prefix itself might contain special characters (like % or _).
Is there a way to escape the prefix, but not the trailing _ without rolling my own sanitizer with a bunch of #gsubs?
There is no better solution than replacing all _ with \_ and all % with \% to escape their special meaning.
Model.where("field LIKE ?||'_'", escapeDataFunction("#{prefix}")).count
The idea is to escape what needs to be escaped and hard code the other part in the "where" condition. Also note that when using substitution variables (? or :1), then the data need not be escaped at all in general, but "like" expressions are an exception, and in that case, you should escape the special characters with meaning in the like operator.
Does anyone understand what this (([A-Za-z\\s])+)\\? means?
I wonder why it should be "\\s" and "\\" ?
If I entered "\s", Xcode just doesn't understand and if I entered "\?", it just doesn't match the "?".
I have googled a lot, but I did not find a solution. Anyone knows?
The actual regex is (([A-Za-z\s])+)\?. This matches one or more letters and whitespace characters followed by an question mark. The \ has two different meanings here. In the first instance \s has a fixed meaning and stands for any white space characters. In the second instance the \? means the literal question mark character. The escaping is necessary as the question mark means one or none of the previous otherwise.
You can't type your regex like this in a string literal in C code though. C also does some escaping using the backslash character. For example "\n" is translated to a string containing only a newline character. There are some other escape sequences with special meanings. If the character after the backslash doesn't have a special meaning the backslash is just removed. That means if you want to have a single backspace in your string you have to write two.
So if you wrote your regex string as you wanted you'd get different results as it would be interpreted as (([A-Za-zs])+)? which has a completely different meaning. So when you write a regex in an ObjC (or any other C-based language) string literal you must double all backslash characters.
not sure about ios but same thing happens in java. \ is escape character for java,and c also so when you type \s java reads \ as an escape character.
think of it as if you want to print a \ what will you have to do.
you will have to type \\. now first \ will work as escape character for java and second one will be printed.
I think it should be the same concept for ios too.
so if you want \s you type \s, if you want \ you type \\.
The \s metacharacter is used to find a whitespace character.
Refer this!
I want to define an array in ruby in following manner
A = ["\"]
I am stuck here for hours now. Tried several possible combinations of single and double quotes, forward and backward slashes. Alas !!
I have seen this link as well : here
But couldn't understand how to resolve my problem.
Apart from this what I need to do is -
1. Read a file character by character (which I managed to do !)
2. This file contains a "\" character
3. I want to do something if my array A includes this backslash
A.includes?("\")
Any help appreciated !
There are some characters which are special and need to be escaped.
Like when you define a string
str = " this is test string \
and this contains multiline data \
do you understand the backslash meaning here \
it is being used to denote the continuation of line"
In a string defined in a double quotes "", if you need to have a double quote how would you doo that? "\"", this is why when you put a backslash in a string you are telling interpretor you are going to use some special characters and which are escaped by backslash. So when you read a "\" from a file it will be read as "\" this into a ruby string.
char = "\\"
char.length # => 1
I hope this helps ;)
Your issue is not with Array, your question really involves escape sequences for special characters in strings. As the \ character is special, you need to first prepend it (escape it) with a leading backslash, like so.
"\\"
You should also re-read your link and the section on escape sequences.
You can escape backslash with a backslash in double quotes like:
["\\"].include?("\\")