I am tired of always trying to guess, if I should escape special characters like '()[]{}|' etc. when using many implementations of regexps.
It is different with, for example, Python, sed, grep, awk, Perl, rename, Apache, find and so on.
Is there any rule set which tells when I should, and when I should not, escape special characters? Does it depend on the regexp type, like PCRE, POSIX or extended regexps?
Which characters you must and which you mustn't escape indeed depends on the regex flavor you're working with.
For PCRE, and most other so-called Perl-compatible flavors, escape these outside character classes:
.^$*+?()[{\|
and these inside character classes:
^-]\
For POSIX extended regexes (ERE), escape these outside character classes (same as PCRE):
.^$*+?()[{\|
Escaping any other characters is an error with POSIX ERE.
Inside character classes, the backslash is a literal character in POSIX regular expressions. You cannot use it to escape anything. You have to use "clever placement" if you want to include character class metacharacters as literals. Put the ^ anywhere except at the start, the ] at the start, and the - at the start or the end of the character class to match these literally, e.g.:
[]^-]
In POSIX basic regular expressions (BRE), these are metacharacters that you need to escape to suppress their meaning:
.^$*[\
Escaping parentheses and curly brackets in BREs gives them the special meaning their unescaped versions have in EREs. Some implementations (e.g. GNU) also give special meaning to other characters when escaped, such as \? and +. Escaping a character other than .^$*(){} is normally an error with BREs.
Inside character classes, BREs follow the same rule as EREs.
If all this makes your head spin, grab a copy of RegexBuddy. On the Create tab, click Insert Token, and then Literal. RegexBuddy will add escapes as needed.
Modern RegEx Flavors (PCRE)
Includes C, C++, Delphi, EditPad, Java, JavaScript, Perl, PHP (preg), PostgreSQL, PowerGREP, PowerShell, Python, REALbasic, Real Studio, Ruby, TCL, VB.Net, VBScript, wxWidgets, XML Schema, Xojo, XRegExp.PCRE compatibility may vary
Anywhere: . ^ $ * + - ? ( ) [ ] { } \ |
Legacy RegEx Flavors (BRE/ERE)
Includes awk, ed, egrep, emacs, GNUlib, grep, PHP (ereg), MySQL, Oracle, R, sed.PCRE support may be enabled in later versions or by using extensions
ERE/awk/egrep/emacs
Outside a character class: . ^ $ * + ? ( ) [ { } \ |
Inside a character class: ^ - [ ]
BRE/ed/grep/sed
Outside a character class: . ^ $ * [ \
Inside a character class: ^ - [ ]
For literals, don't escape: + ? ( ) { } |
For standard regex behavior, escape: \+ \? \( \) \{ \} \|
Notes
If unsure about a specific character, it can be escaped like \xFF
Alphanumeric characters cannot be escaped with a backslash
Arbitrary symbols can be escaped with a backslash in PCRE, but not BRE/ERE (they must only be escaped when required). For PCRE ] - only need escaping within a character class, but I kept them in a single list for simplicity
Quoted expression strings must also have the surrounding quote characters escaped, and often with backslashes doubled-up (like "(\")(/)(\\.)" versus /(")(\/)(\.)/ in JavaScript)
Aside from escapes, different regex implementations may support different modifiers, character classes, anchors, quantifiers, and other features. For more details, check out regular-expressions.info, or use regex101.com to test your expressions live
Unfortunately there really isn't a set set of escape codes since it varies based on the language you are using.
However, keeping a page like the Regular Expression Tools Page or this Regular Expression Cheatsheet can go a long way to help you quickly filter things out.
POSIX recognizes multiple variations on regular expressions - basic regular expressions (BRE) and extended regular expressions (ERE). And even then, there are quirks because of the historical implementations of the utilities standardized by POSIX.
There isn't a simple rule for when to use which notation, or even which notation a given command uses.
Check out Jeff Friedl's Mastering Regular Expressions book.
Unfortunately, the meaning of things like ( and \( are swapped between Emacs style regular expressions and most other styles. So if you try to escape these you may be doing the opposite of what you want.
So you really have to know what style you are trying to quote.
Really, there isn't. there are about a half-zillion different regex syntaxes; they seem to come down to Perl, EMACS/GNU, and AT&T in general, but I'm always getting surprised too.
Sometimes simple escaping is not possible with the characters you've listed. For example, using a backslash to escape a bracket isn't going to work in the left hand side of a substitution string in sed, namely
sed -e 's/foo\(bar/something_else/'
I tend to just use a simple character class definition instead, so the above expression becomes
sed -e 's/foo[(]bar/something_else/'
which I find works for most regexp implementations.
BTW Character classes are pretty vanilla regexp components so they tend to work in most situations where you need escaped characters in regexps.
Edit: After the comment below, just thought I'd mention the fact that you also have to consider the difference between finite state automata and non-finite state automata when looking at the behaviour of regexp evaluation.
You might like to look at "the shiny ball book" aka Effective Perl (sanitised Amazon link), specifically the chapter on regular expressions, to get a feel for then difference in regexp engine evaluation types.
Not all the world's a PCRE!
Anyway, regexp's are so clunky compared to SNOBOL! Now that was an interesting programming course! Along with the one on Simula.
Ah the joys of studying at UNSW in the late '70's! (-:
https://perldoc.perl.org/perlre.html#Quoting-metacharacters and https://perldoc.perl.org/functions/quotemeta.html
In the official documentation, such characters are called metacharacters. Example of quoting:
my $regex = quotemeta($string)
s/$regex/something/
For PHP, "it is always safe to precede a non-alphanumeric with "\" to specify that it stands for itself." - http://php.net/manual/en/regexp.reference.escape.php.
Except if it's a " or '. :/
To escape regex pattern variables (or partial variables) in PHP use preg_quote()
To know when and what to escape without attempts is necessary to understand precisely the chain of contexts the string pass through. You will specify the string from the farthest side to its final destination which is the memory handled by the regexp parsing code.
Be aware how the string in memory is processed: if can be a plain string inside the code, or a string entered to the command line, but a could be either an interactive command line or a command line stated inside a shell script file, or inside a variable in memory mentioned by the code, or an (string)argument through further evaluation, or a string containing code generated dynamically with any sort of encapsulation...
Each of this context assigned some characters with special functionality.
When you want to pass the character literally without using its special function (local to the context), than that's the case you have to escape it, for the next context... which might need some other escape characters which might additionally need to be escaped in the preceding context(s).
Furthermore there can be things like character encoding (the most insidious is utf-8 because it look like ASCII for common characters, but might be optionally interpreted even by the terminal depending on its settings so it might behave differently, then the encoding attribute of HTML/XML, it's necessary to understand the process precisely right.
E.g. A regexp in the command line starting with perl -npe, needs to be transferred to a set of exec system calls connecting as pipe the file handles, each of this exec system calls just has a list of arguments that were separated by (non escaped)spaces, and possibly pipes(|) and redirection (> N> N>&M), parenthesis, interactive expansion of * and ?, $(()) ... (all this are special characters used by the *sh which might appear to interfere with the character of the regular expression in the next context, but they are evaluated in order: before the command line. The command line is read by a program as bash/sh/csh/tcsh/zsh, essentially inside double quote or single quote the escape is simpler but it is not necessary to quote a string in the command line because mostly the space has to be prefixed with backslash and the quote are not necessary leaving available the expand functionality for characters * and ?, but this parse as different context as within quote. Then when the command line is evaluated the regexp obtained in memory (not as written in the command line) receives the same treatment as it would be in a source file.
For regexp there is character-set context within square brackets [ ], perl regular expression can be quoted by a large set of non alfa-numeric characters (E.g. m// or m:/better/for/path: ...).
You have more details about characters in other answer, which are very specific to the final regexp context. As I noted you mention that you find the regexp escape with attempts, that's probably because different context has different set of character that confused your memory of attempts (often backslash is the character used in those different context to escape a literal character instead of its function).
For Ionic (Typescript) you have to double slash in order to scape the characters.
For example (this is to match some special characters):
"^(?=.*[\\]\\[!¡\'=ªº\\-\\_ç##$%^&*(),;\\.?\":{}|<>\+\\/])"
Pay attention to this ] [ - _ . / characters. They have to be double slashed. If you don't do that, you are going to have a type error in your code.
to avoid having to worry about which regex variant and all the bespoke peculiarties, just use this generic function that covers every regex variant other than BRE (unless they have unicode multi-byte chars that are meta) :
jot -s '' -c - 32 126 |
mawk '
function ___(__,_) {
return substr(_="",
gsub("[][!-/_\140:-#{-~]","[&]",__),
gsub("["(_="\\\\")"^]",_ "&",__))__
} ($++NF = ___($!_))^_'
!"#$%&'()*+,-./0123456789:;<=>?
#ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_
`abcdefghijklmnopqrstuvwxyz{|}~
[!]["][#][$][%][&]['][(][)][*][+][,][-][.][/]
0 1 2 3 4 5 6 7 8 9 [:][;][<][=][>][?]
[#] ABCDEFGHIJKLMNOPQRSTUVWXYZ [[]\\ []]\^ [_]
[`] abcdefghijklmnopqrstuvwxyz [{][|][}][~]
square-brackets are much easier to deal with, since there's no risk of triggering warning messages about "escaping too much", e.g. :
function ____(_) {
return substr("", gsub("[[:punct:]]","\\\\&",_))_
}
\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/ 0123456789\:\;\<\=\>\?
\#ABCDEFGHIJKLMNOPQRSTUVWXYZ\[\\\]\^\_\`abcdefghijklmnopqrstuvwxyz \{\|\}\~
gawk: cmd. line:1: warning: regexp escape sequence `\!' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\"' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\#' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\%' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\&' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\,' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\:' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\;' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\=' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\#' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\_' is not a known regexp operator
gawk: cmd. line:1: warning: regexp escape sequence `\~' is not a known regexp operator
Using Raku (formerly known as Perl_6)
Works (backslash or quote all non-alphanumeric characters except underscore):
~$ raku -e 'say $/ if "#.*?" ~~ m/ \# \. \* \? /; #works fine'
｢#.*?｣
There exist six flavors of Regular Expression languages, according to Damian Conway's pdf/talk "Everything You Know About Regexes Is Wrong". Raku represents a significant (~15 year) re-working of standard Perl(5)/PCRE Regular Expressions.
In those 15 years the Perl_6 / Raku language experts decided that all non-alphanumeric characters (except underscore) shall be reserved as Regex metacharacters even if no present usage exists. To denote non-alphanumeric characters (except underscore) as literals, backslash or escape them.
So the above example prints the $/ match variable if a match to a literal #.*? character sequence is found. Below is what happens if you don't: # is interpreted as the start of a comment, . dot is interpreted as any character (including whitespace), * asterisk is interpreted as a zero-or-more quantifier, and ? question mark is interpreted as either a zero-or-one quantifier or a frugal (i.e. non-greedy) quantifier-modifier (depending on context):
Errors:
~$ ~$ raku -e 'say $/ if "#.*?" ~~ m/ # . * ? /; #ERROR!'
===SORRY!===
Regex not terminated.
at -e:1
------> y $/ if "#.*?" ~~ m/ # . * ? /; #ERROR!⏏<EOL>
Regex not terminated.
at -e:1
------> y $/ if "#.*?" ~~ m/ # . * ? /; #ERROR!⏏<EOL>
Couldn't find terminator / (corresponding / was at line 1)
at -e:1
------> y $/ if "#.*?" ~~ m/ # . * ? /; #ERROR!⏏<EOL>
expecting any of:
/
https://docs.raku.org/language/regexes
https://raku.org/
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I don't really understand regular expressions. Can you explain them to me in an easy-to-follow manner? If there are any online tools or books, could you also link to them?
The most important part is the concepts. Once you understand how the building blocks work, differences in syntax amount to little more than mild dialects. A layer on top of your regular expression engine's syntax is the syntax of the programming language you're using. Languages such as Perl remove most of this complication, but you'll have to keep in mind other considerations if you're using regular expressions in a C program.
If you think of regular expressions as building blocks that you can mix and match as you please, it helps you learn how to write and debug your own patterns but also how to understand patterns written by others.
Start simple
Conceptually, the simplest regular expressions are literal characters. The pattern N matches the character 'N'.
Regular expressions next to each other match sequences. For example, the pattern Nick matches the sequence 'N' followed by 'i' followed by 'c' followed by 'k'.
If you've ever used grep on Unix—even if only to search for ordinary looking strings—you've already been using regular expressions! (The re in grep refers to regular expressions.)
Order from the menu
Adding just a little complexity, you can match either 'Nick' or 'nick' with the pattern [Nn]ick. The part in square brackets is a character class, which means it matches exactly one of the enclosed characters. You can also use ranges in character classes, so [a-c] matches either 'a' or 'b' or 'c'.
The pattern . is special: rather than matching a literal dot only, it matches any character†. It's the same conceptually as the really big character class [-.?+%$A-Za-z0-9...].
Think of character classes as menus: pick just one.
Helpful shortcuts
Using . can save you lots of typing, and there are other shortcuts for common patterns. Say you want to match a digit: one way to write that is [0-9]. Digits are a frequent match target, so you could instead use the shortcut \d. Others are \s (whitespace) and \w (word characters: alphanumerics or underscore).
The uppercased variants are their complements, so \S matches any non-whitespace character, for example.
Once is not enough
From there, you can repeat parts of your pattern with quantifiers. For example, the pattern ab?c matches 'abc' or 'ac' because the ? quantifier makes the subpattern it modifies optional. Other quantifiers are
* (zero or more times)
+ (one or more times)
{n} (exactly n times)
{n,} (at least n times)
{n,m} (at least n times but no more than m times)
Putting some of these blocks together, the pattern [Nn]*ick matches all of
ick
Nick
nick
Nnick
nNick
nnick
(and so on)
The first match demonstrates an important lesson: * always succeeds! Any pattern can match zero times.
A few other useful examples:
[0-9]+ (and its equivalent \d+) matches any non-negative integer
\d{4}-\d{2}-\d{2} matches dates formatted like 2019-01-01
Grouping
A quantifier modifies the pattern to its immediate left. You might expect 0abc+0 to match '0abc0', '0abcabc0', and so forth, but the pattern immediately to the left of the plus quantifier is c. This means 0abc+0 matches '0abc0', '0abcc0', '0abccc0', and so on.
To match one or more sequences of 'abc' with zeros on the ends, use 0(abc)+0. The parentheses denote a subpattern that can be quantified as a unit. It's also common for regular expression engines to save or "capture" the portion of the input text that matches a parenthesized group. Extracting bits this way is much more flexible and less error-prone than counting indices and substr.
Alternation
Earlier, we saw one way to match either 'Nick' or 'nick'. Another is with alternation as in Nick|nick. Remember that alternation includes everything to its left and everything to its right. Use grouping parentheses to limit the scope of |, e.g., (Nick|nick).
For another example, you could equivalently write [a-c] as a|b|c, but this is likely to be suboptimal because many implementations assume alternatives will have lengths greater than 1.
Escaping
Although some characters match themselves, others have special meanings. The pattern \d+ doesn't match backslash followed by lowercase D followed by a plus sign: to get that, we'd use \\d\+. A backslash removes the special meaning from the following character.
Greediness
Regular expression quantifiers are greedy. This means they match as much text as they possibly can while allowing the entire pattern to match successfully.
For example, say the input is
"Hello," she said, "How are you?"
You might expect ".+" to match only 'Hello,' and will then be surprised when you see that it matched from 'Hello' all the way through 'you?'.
To switch from greedy to what you might think of as cautious, add an extra ? to the quantifier. Now you understand how \((.+?)\), the example from your question works. It matches the sequence of a literal left-parenthesis, followed by one or more characters, and terminated by a right-parenthesis.
If your input is '(123) (456)', then the first capture will be '123'. Non-greedy quantifiers want to allow the rest of the pattern to start matching as soon as possible.
(As to your confusion, I don't know of any regular-expression dialect where ((.+?)) would do the same thing. I suspect something got lost in transmission somewhere along the way.)
Anchors
Use the special pattern ^ to match only at the beginning of your input and $ to match only at the end. Making "bookends" with your patterns where you say, "I know what's at the front and back, but give me everything between" is a useful technique.
Say you want to match comments of the form
-- This is a comment --
you'd write ^--\s+(.+)\s+--$.
Build your own
Regular expressions are recursive, so now that you understand these basic rules, you can combine them however you like.
Tools for writing and debugging regexes:
RegExr (for JavaScript)
Perl: YAPE: Regex Explain
Regex Coach (engine backed by CL-PPCRE)
RegexPal (for JavaScript)
Regular Expressions Online Tester
Regex Buddy
Regex 101 (for PCRE, JavaScript, Python, Golang, Java 8)
I Hate Regex
Visual RegExp
Expresso (for .NET)
Rubular (for Ruby)
Regular Expression Library (Predefined Regexes for common scenarios)
Txt2RE
Regex Tester (for JavaScript)
Regex Storm (for .NET)
Debuggex (visual regex tester and helper)
Books
Mastering Regular Expressions, the 2nd Edition, and the 3rd edition.
Regular Expressions Cheat Sheet
Regex Cookbook
Teach Yourself Regular Expressions
Free resources
RegexOne - Learn with simple, interactive exercises.
Regular Expressions - Everything you should know (PDF Series)
Regex Syntax Summary
How Regexes Work
JavaScript Regular Expressions
Footnote
†: The statement above that . matches any character is a simplification for pedagogical purposes that is not strictly true. Dot matches any character except newline, "\n", but in practice you rarely expect a pattern such as .+ to cross a newline boundary. Perl regexes have a /s switch and Java Pattern.DOTALL, for example, to make . match any character at all. For languages that don't have such a feature, you can use something like [\s\S] to match "any whitespace or any non-whitespace", in other words anything.
I have a regular expression in php and I need to convert it to java.
Is it possible to do so? If yes how can i do?
Thanks in advance
$region_pattern = "/<a href=\"#\"><img src=\"images\/ponto_[^\.]+\.gif\"[^>]*>[ ]*<strong>(?P<neighborhood>[^\(<]+)\((?P<region>[^\)]+)\)<\/strong><\/a>/i" ;
A typical conversion from any regex to java is to:
Exclude pattern delimiters => remove starting and trailing /
Remove flags, these are applied to the Pattern object, this is the trailing i. You should either put it in the initialisation of your Pattern object or prepend it to the regex like (?i)<regex>
Replace all \ with \\, \ has a meaning already in java(escape in strings), to use a backslash inside a regex in java you have to use \\ instead of \, so \w becomes \\w. and \\ becomes \\\\
Above regex would become
Pattern.compile("<a href=\"#\"><img src=\"images\\/ponto_[^\\.]+\\.gif\"[^>]*>[ ]*<strong>(?P<neighborhood>[^\\(<]+)\\((?P<region>[^\\)]+)\\)<\\/strong><\\/a>", Pattern.CASE_INSENSITIVE);
This will fail however, I think it is because ?P is a modifier, not one I know exists in Java so ye it is a invalid regex.
There are some problems with the original regex that have to be cleared away first. First, there's [ ], which matches one of the characters &, n, b, s, p or ;. To match an actual non-breaking space character, you should use \xA0.
You also have a lot of unneeded backslashes in there. You can get rid of some by changing the regex delimiter to something other than /; others aren't needed because they're inside character classes, where most metacharacters lose their special meanings. That leaves you with this PHP regex:
"~<img src=\"images/ponto_[^.]+\.gif\"[^>]*>\xA0*<strong>(?P<neighborhood>[^(<]+)\((?P<region>[^)]+)\)</strong>~i"
There are three things that make this regex incompatible with Java. One is the delimiters (/ originally, ~ in the version above) along with the trailing i modifier. Java doesn't use regex delimiters at all, so just drop those. The modifier can be moved into the regex itself by using the inline form, (?i), at the beginning of the regex. (That will work in PHP too, by the way.)
Next is the backslashes. The ones that are used to escape quotation marks remain as they are, but all the others get doubled because Java is more strict about escape sequences in string literals.
Finally, there are the named groups. Up until Java 6, named groups weren't supported at all; Java 7 supports them, but they use the shorter (?<name>...) syntax favored by .NET,
not the Pythonesque (?P<name>...) syntax. (By the way, the shorter (?<name>...) version should work in PHP, too (as should (?'name'...), also introduced by .NET).
So the Java 7 version of your regex would be:
"(?i)<img src=\"images/ponto_[^.]+\\.gif\"[^>]*>\\xA0*<strong>(?<neighborhood>[^(<]+)\\((?<region>[^)]+)\\)</strong>"
For Java 6 or earlier you would use:
"(?i)<img src=\"images/ponto_[^.]+\\.gif\"[^>]*>\\xA0*<strong>([^(<]+)\\(([^)]+)\\)</strong>"
...and you'd have to use numbers instead of names to refer to the group captures.
REGEX is REGEX regardless of language. The REGEX you've posted will work on both Java and PHP. You do need to make some adjustments as both language don't take the pattern exactly the same (though the pattern itself will work in both languages).
Points to Consider
You should know that Java's Pattern object applies flags without having to specify them on the pattern string itself.
Delimiters should not be included as well. Only the pattern itself.
There are many questions and answers here on StackOverflow that assume a "letter" can be matched in a regexp by [a-zA-Z]. However with Unicode there are many more characters that most people would regard as a letter (all the Greek letters, Cyrllic .. and many more. Unicode defines many blocks each of which may have "letters".
The Java definition defines Posix classes for things like alpha characters, but that is specified to only work with US-ASCII. The predefined character classes define words to consist of [a-zA-Z_0-9], which also excludes many letters.
So how do you properly match against Unicode strings? Is there some other library that gets this right?
Here you have a very nice explanation:
http://www.regular-expressions.info/unicode.html
Some hints:
Java and .NET unfortunately do not support \X (yet). Use \P{M}\p{M}* as a substitute. To match any number of graphemes, use (?:\P{M}\p{M}*)+ instead of \X+.
In Java, the regex token \uFFFF only matches the specified code point, even when you turned on canonical equivalence. However, the same syntax \uFFFF is also used to insert Unicode characters into literal strings in the Java source code. Pattern.compile("\u00E0") will match both the single-code-point and double-code-point encodings of à, while Pattern.compile("\\u00E0") matches only the single-code-point version. Remember that when writing a regex as a Java string literal, backslashes must be escaped. The former Java code compiles the regex à, while the latter compiles \u00E0. Depending on what you're doing, the difference may be significant.
Are you talking about Unicode categories, like letters? These are matched by a regex of the form \p{CAT}, where "CAT" is the category code like L for any letter, or a subcategory like Lu for uppercase or Lt for title-case.
Quoting from the JavaDoc of java.util.regex.Pattern.
Unicode support
This class is in conformance with
Level 1 of Unicode Technical Standard #18: Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents.
Unicode escape sequences such as
\u2014 in Java source code are
processed as described in §3.3 of the
Java Language Specification. Such
escape sequences are also implemented
directly by the regular-expression
parser so that Unicode escapes can be
used in expressions that are read from
files or from the keyboard. Thus the
strings "\u2014" and "\\u2014", while
not equal, compile into the same
pattern, which matches the character
with hexadecimal value 0x2014.
Unicode blocks and categories are
written with the \p and \P constructs
as in Perl. \p{prop} matches if the
input has the property prop, while
\P{prop} does not match if the input
has that property. Blocks are
specified with the prefix In, as in
InMongolian. Categories may be
specified with the optional prefix Is:
Both \p{L} and \p{IsL} denote the
category of Unicode letters. Blocks
and categories can be used both inside
and outside of a character class.
The supported categories are those of
The Unicode Standard in the version
specified by the Character class. The
category names are those defined in
the Standard, both normative and
informative. The block names supported
by Pattern are the valid block names
accepted and defined by
UnicodeBlock.forName.
Which regular expression engine does Java uses?
In a tool like RegexBuddy if I use
[a-z&&[^bc]]
that expression in Java is good but in RegexBuddy it has not been understood.
In fact it reports:
Match a single character present in
the list below [a-z&&[^bc]
A character in the range between a and z : a-z
One of the characters &[^bc : &&[^bc
Match the character ] literally : ]
but i want to match a character between a and z intersected with a character that is not b or c
Like most regex flavors, java.util.regex.Pattern has its own specific features with syntax that may not be fully compatible with others; this includes character class union, intersection and subtraction:
[a-d[m-p]] : a through d, or m through p: [a-dm-p] (union)
[a-z&&[def]] : d, e, or f (intersection)
[a-z&&[^bc]] : a through z, except for b and c: [ad-z] (subtraction)
The most important "caveat" of Java regex is that matches attempts to match a pattern against the whole string. This is atypical of most engines, and can be a source of confusion at times.
See also
regular-expressions.info/Flavor Comparison and Java Flavor Notes
On character class subtraction
Subtraction allows you to define for example "all consonants" in Java as [a-z&&[^aeiou]].
This syntax is specific to Java. In XML Schema, .NET, JGSoft and RegexBuddy, it's [a-z-[aeiou]]. Other flavors may not support this feature at all.
References
regular-expressions.info/Character Classes in XML Regular Expressions
MSDN - Regular Expression Character Classes - Subtraction
Related questions
What is the point behind character class intersections in Java’s Regex?
Java uses its own regular expression engine, which behaviour is defined in the Pattern class.
You can test it with an Eclipse plugin or online.
RegexBuddy does not yet support the character class union, intersection, and subtraction syntax that is unique to the Java regular expression flavor. This is the only part of the Java regex syntax that RegexBuddy does not yet support. We're planning to implement this in a future version of RegexBuddy. The reason this has been postponed is because no other regular expression flavor supports this syntax.
P.S.: If you have a question about RegexBuddy in particular, please add the "regexbuddy" tag to your question. Then the question automatically shows up in my RSS reader. I don't follow the "regex" tag because far too many questions use that tag, and most are already answered by the time I see them.