I have to replace all occurrences of '\''\' with '\' in a String. I know regex "\\\\" means \ but how do I write a regex for replaceAll() to match '\''\'. I tried:
.replaceAll("\\\\\\\\", "\\")
but I get a java.util.regex.PatternSyntaxException?
If you want to replace literals instead of replaceAll use replace method which doesn't use regex syntax:
replace("\\\\", "\\")
If you absolutely must use replaceAll remember that its second parameter also have some special characters which are
$ (where $x represents match from group x)
and \ to escape that $ and itself,
so code using replaceAll would need to look like:
replaceAll("\\\\\\\\", "\\\\")
since we also need to escape \ twice (once in regex engine \\, once in string \\\\).
Related
How to write a regular expression to match this \" (a backslash then a quote)? Assume I have a string like this:
click to search
I need to replace all the \" with a ", so the result would look like:
click to search
This one does not work: str.replaceAll("\\\"", "\"") because it only matches the quote. Not sure how to get around with the backslash. I could have removed the backslash first, but there are other backslashes in my string.
If you don't need any of regex mechanisms like predefined character classes \d, quantifiers etc. instead of replaceAll which expects regex use replace which expects literals
str = str.replace("\\\"","\"");
Both methods will replace all occurrences of targets, but replace will treat targets literally.
BUT if you really must use regex you are looking for
str = str.replaceAll("\\\\\"", "\"")
\ is special character in regex (used for instance to create \d - character class representing digits). To make regex treat \ as normal character you need to place another \ before it to turn off its special meaning (you need to escape it). So regex which we are trying to create is \\.
But to create string literal representing text \\ so you could pass it to regex engine you need to write it as four \ ("\\\\"), because \ is also special character in String literals (part of code written using "...") since it can be used for instance as \t to represent tabulator.
That is why you also need to escape \ there.
In short you need to escape \ twice:
in regex \\
and then in String literal "\\\\"
You don't need a regular expression.
str.replace("\\\"", "\"")
should work just fine.
The replace method takes two substrings and replaces all non-overlapping occurrences of the first with the second. Per the javadoc:
public String replace(CharSequence target,
CharSequence replacement)
Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".
try this: str.replaceAll("\\\\\"", "\\\"")
because Java will replace \ twice:
(1) \\\\\" --> \\" (for string)
(2) \\" --> \" (for regex)
How to write a regular expression to match this \" (a backslash then a quote)? Assume I have a string like this:
click to search
I need to replace all the \" with a ", so the result would look like:
click to search
This one does not work: str.replaceAll("\\\"", "\"") because it only matches the quote. Not sure how to get around with the backslash. I could have removed the backslash first, but there are other backslashes in my string.
If you don't need any of regex mechanisms like predefined character classes \d, quantifiers etc. instead of replaceAll which expects regex use replace which expects literals
str = str.replace("\\\"","\"");
Both methods will replace all occurrences of targets, but replace will treat targets literally.
BUT if you really must use regex you are looking for
str = str.replaceAll("\\\\\"", "\"")
\ is special character in regex (used for instance to create \d - character class representing digits). To make regex treat \ as normal character you need to place another \ before it to turn off its special meaning (you need to escape it). So regex which we are trying to create is \\.
But to create string literal representing text \\ so you could pass it to regex engine you need to write it as four \ ("\\\\"), because \ is also special character in String literals (part of code written using "...") since it can be used for instance as \t to represent tabulator.
That is why you also need to escape \ there.
In short you need to escape \ twice:
in regex \\
and then in String literal "\\\\"
You don't need a regular expression.
str.replace("\\\"", "\"")
should work just fine.
The replace method takes two substrings and replaces all non-overlapping occurrences of the first with the second. Per the javadoc:
public String replace(CharSequence target,
CharSequence replacement)
Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".
try this: str.replaceAll("\\\\\"", "\\\"")
because Java will replace \ twice:
(1) \\\\\" --> \\" (for string)
(2) \\" --> \" (for regex)
Consider the string,
this\is\\a\new\\string
The output should be:
this\is\a\new\string
So basically one or more \ character should be replaced with just one \.
I tried the following:
str = str.replace("[\\]+","\")
but it was no use. The reason I used two \ in [\\]+ was because internally \ is stored as \\. I know this might be a basic regex question, but I am able to replace one or more normal alphabets but not \ character. Any help is really appreciated.
str.replace("[\\]+", "\") has few problems,
replace doesn't use regex (replaceAll does) so "[\\]" will represent [\] literal, not \ nor \\ (depending on what you think it would represent)
even if it did accept regex "[\\]" would not be correct regex because \\] would escape ] so you would end up with unclosed character class [..
it will not compile (your replacement String is not closed)
It will not compile because \ is start of escape sequence \X where X needs to be either
changed from being String special character to simple literal, like in your case \" will escape " to be literal (so you could print it for instance) instead of being start/end of String,
changed from being normal character to be special one like in case of line separators \n \r or tabulations \t.
Now we know that \ is special and is used to escape other character. So what do you think we need to do to make \ represent literal (when we want to print \). If you guessed that it needs to be escaped with another \ then you are right. To create \ literal we need to write it in String as "\\".
Since you know how to create String containing \ literal (escaped \) you can start thinking about how to create your replacements.
Regex which represents one or more \ can look like
\\+
But that is its native form, and we need to create it using String. I used \\ here because in regex \ is also special character (for instance \d represents digits , not \ literal followed by d) so it also needs to be escaped first to represent \ literal. Just like in String we can escape it with another \.
So String representing this regex will need to be written as
"\\\\+" (we escaped \ twice, once in regex \\+ and once in string)
You can use it as first argument of replaceAll (because replace as mentioned earlier doesn't accept regex).
Now last problem you will face is second argument of replaceAll method. If you write
replaceAll("\\\\+", "\\")
and it will find match for regex you will see exception
java.lang.IllegalArgumentException: character to be escaped is missing
It is because in replacement part (second argument in replaceAll method) we can also use special formula $x which represents current match from group with index x. So to be able to escape $ into literal we need some escape mechanism, and again \ was used here for that purpose. So \ is also special in replacement part of our method.
So again to create \ literal we need to escape it with another \, and string literal representing expression \\ is "\\\\".
But lets get back to earlier exception: message "character to be escaped is missing" refers to X part of \X formula (X is character we want to be escaped). Problem is that earlier your replacement "\\" represented only \ part, so this method expected either $ to create \$, or \\ to create \ literal. So valid replacements would be "\\$ or "\\\\".
To make things work you need to write your replacing method as
str = str.replaceAll("\\\\+", "\\\\")
You can use:
str = str.replace("\\\\", "\\");
Remember that String#replace doesn't take a regex.
try this
str = str.replaceAll("\\\\+", "\\\\");
When writing regular expressions, you typically need to double-escape backslashes. So you would do this:
str = str.replaceAll("\\\\+", "\\\\");
I'd use Matcher.quoteReplacement() and String.replaceAll() here.
Like this:
String s;
[...]
s = s.replaceAll("\\\\+", Matcher.quoteReplacement("\\"));
How to write a regular expression to match this \" (a backslash then a quote)? Assume I have a string like this:
click to search
I need to replace all the \" with a ", so the result would look like:
click to search
This one does not work: str.replaceAll("\\\"", "\"") because it only matches the quote. Not sure how to get around with the backslash. I could have removed the backslash first, but there are other backslashes in my string.
If you don't need any of regex mechanisms like predefined character classes \d, quantifiers etc. instead of replaceAll which expects regex use replace which expects literals
str = str.replace("\\\"","\"");
Both methods will replace all occurrences of targets, but replace will treat targets literally.
BUT if you really must use regex you are looking for
str = str.replaceAll("\\\\\"", "\"")
\ is special character in regex (used for instance to create \d - character class representing digits). To make regex treat \ as normal character you need to place another \ before it to turn off its special meaning (you need to escape it). So regex which we are trying to create is \\.
But to create string literal representing text \\ so you could pass it to regex engine you need to write it as four \ ("\\\\"), because \ is also special character in String literals (part of code written using "...") since it can be used for instance as \t to represent tabulator.
That is why you also need to escape \ there.
In short you need to escape \ twice:
in regex \\
and then in String literal "\\\\"
You don't need a regular expression.
str.replace("\\\"", "\"")
should work just fine.
The replace method takes two substrings and replaces all non-overlapping occurrences of the first with the second. Per the javadoc:
public String replace(CharSequence target,
CharSequence replacement)
Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".
try this: str.replaceAll("\\\\\"", "\\\"")
because Java will replace \ twice:
(1) \\\\\" --> \\" (for string)
(2) \\" --> \" (for regex)
I'm trying to escape a RegExp metacharacter in Java. Below is what I want:
INPUT STRING: "This is $ test"
OUTPUT STRING: "This is \$ test"
This is what I'm currently doing but it's not working:
String inputStr= "This is $ test";
inputStr = inputStr.replaceAll("$","\\$");
But I'm getting wrong output:
"This is $ test$"
You'll need:
inputStr.replaceAll("\\$", "\\\\\\$");
The String to be replaced needs 2 backslashes because $ has a special meaning in the regexp. So $ must be escaped, to get: \$, and that backslash must itself be escaped within the java String: "\\$".
The replacement string needs 6 backslashes because both \ and $ have special meaning in the replacement strings:
\ can be used to escape characters in the replacement string.
$ can be used to make back-references in the replacement string.
So if your intended replacement string is "\$", you need to escape each of those two characters to get: \\\$, and then each backslash you need to use - 3 of them, 1 literal and 2 for escapes - must also be escaped within the java String: "\\\\\\$".
See: Matcher.replaceAll
As you said, $ is a reserved character for Regex. Then, you need to escape it. You can use a backslash character to do this:
inputStr.replaceAll("\\$", ...);
In the replacement, the $ and \ characters also have a special meaning:
Note that backslashes () and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string; see Matcher.replaceAll
Then, the replacement will be the backslash character and the dollar sign, both of them being escaped by a '\' character (which needs to be doubled toi build the String):
inputStr.replaceAll("\\$", "\\\\\\$");
You have to put 6 backslashes so you escape the backslash and escape the metachar:
inputStr.replaceAll("\\$","\\\\\\$");
The first argument to replaceAll is infact a regexp, and the $ actually means "match the end of the string". You can just use replace instead, which doesn't use regexp, just a normal string replace, to achieve what you want in this case. If you want to use a regexp, just escape the $ in the first argument.