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/
I'm trying to evaluate certain expression. I have "pqr" && "xyz" in command. on evaluating the command, it gives an error: extra characters after close-quote.
I think tcl cant able to parse && after double quote. If this is the reason then how should i have to deal with double quote and &&?
You're not giving us enough information.
A wild guess is that you were writing something like this:
expr "pqr"&& "xyz"
which does give the error message "extra characters after close-quote". This is because the interpreter tries to parse the command according to Tcl language rules, and one of those rules is that a word that starts with a double quote must end with a matching double quote. In this case, there are two & characters following the matching double quote.
Now,
expr "pqr" && "xyz"
(with a space between the double quote and the ampersand) is no good either. This is because the interpreter will remove any characters that have syntactic function as it prepares the arguments for the command. This means that the argument expr gets is the string pqr && xyz. When the expr command executes, it tries to interpret its argument as a string in a special expression language that isn't Tcl. In particular, unlike in Tcl strings that aren't boolean values or the names of variables of functions must always be enclosed in braces or double quotes, like this: "pqr" && "xyz". So how do you get that? You always* brace the argument to expr, that's how.
expr {"pqr" && "xyz"}
means that expr gets the legal string "pqr" && "xyz".
But the string "pqr" && "xyz" is still not valid, since the && (logical and) operation isn't defined for strings other than strings that are equal to the string representation of boolean values, such as expr {"true" && "false"}
So, again we're stuck, because what you seem to be trying to do makes no sense. If you show us what you're doing we might be able to help you.
*) except when you shouldn't. Rare, expert level.
Documentation:
expr,
Mathematical operators as Tcl commands,
Summary of Tcl language syntax
Brace your expressions
The expr command (and by extension, the commands for, if, and while, which use the same mechanism to evaluate their conditions) interprets an expression string that is constructed from its arguments. Note that the language of the expression string isn't Tcl, but specific to the expr command's expression evaluator: the languages share many syntactic forms, but are fundamentally different with infix, operator-based structure for the expr language and prefix, command-based structure for Tcl.
Letting the Tcl interpreter evaluate the arguments before passing them to expr can lead to
double substitution, which has security problems similar to SQL injection attacks.
iterative commands (for, while) getting constant-valued condition arguments, leading to infinite loops.
all substitutions (and thus their side-effects) always occurring while expr can selectively suppress some of them.
Therefore, it is almost always better to provide the expression string as a braced (escaped) string, which will not be evaluated by the Tcl interpreter, only the expr interpreter.
Note that while unbraced arguments to expr are allowed to be a invalid expression string as long as the argument evaluation transforms them into a valid one, braced expressions must be valid as they are (e.g. variable or command substitutions must be simple operands and not operators or complex expressions).
Another benefit from using braced expression strings is that the byte compiler usually can generate more efficient code (5 - 10x faster) from them.
I am trying to do iregex match in django for the regular expression
reg_string = (\w|\d|\b|\s)+h(\w|\d|\b|\s)+(\w|\d|\b|\s)+anto(\w|\d|\b|\s)+
self.queryset.filter(name__iregex=r"%s"%(reg_string,))
by using the word "The Canton" for name but its not returning any value but while using it in python re.search its working
print (re.search(r'(\w|\d|\b|\s)+h(\w|\d|\b|\s)+(\w|\d|\b|\s)+anto(\w|\d|\b|\s)+', 'The Canton', re.I).group()
I am using Mysql 5.7, any one know how to fix this
Note that MySQL REGEXP does not support shorthand character classes like \s, \d, \w, etc. It supports some basic POSIX character classes like [:digit:], [:alpha:], [:alnum:], etc.
Even if you keep on using the pattern in Python, you should not write (\w|\d|\b|\s)+ as it matches and captures a single char that is a word char or digit, word boundary, or whitespace, 1 or more times (and rewriting buffer of Group N with the latest char the engine matched). You could rewrite that with a single character class - [\w\s]+.
Now, your pattern in MySQL will look like
[_[:alnum:][:space:]]+h[_[:alnum:][:space:]]+anto[_[:alnum:][:space:]]+
where [\w\s]+ is turned into [_[:alnum:][:space:]]+:
[ - start of a bracket expression
_ - an underscore (as \w matches _ and [:alnum:] does not)
[:alnum:] - an alphanuemric char
[:space:] - any whitespace char
] - end of the bracket expression
+ - quantifier, 1 or more times.
I want to replace "\cite{foo123a}" with "[1]" and backwards. So far I was able to replace text with the following command
body.replaceText('.cite{foo}', '[1]');
but I did not manage to use
body.replaceText('\cite{foo}', '[1]');
body.replaceText('\\cite{foo}', '[1]');
Why?
The back conversion I cannot get to work at all
body.replaceText('[1]', '\\cite{foo}');
this will replace only the "1" not the [ ], this means the [] are interpreted as regex character set, escaping them will not help
body.replaceText('\[1\]', '\\cite{foo}');//no effect, still a char set
body.replaceText('/\[1\]/', '\\cite{foo}');//no matches
The documentation states
A subset of the JavaScript regular expression features are not fully supported, such as capture groups and mode modifiers.
Can I find a full description of what is supported and what not somewhere?
I'm not familiar with Google Apps Script, but this looks like ordinary regular expression troubles.
Your second conversion is not working because the string literal '\[1\]' is just the same as '[1]'. You want to quote the text \[1\] as a string literal, which means '\\[1\\]'. Slashes inside of a string literal have no relevant meaning; in that case you have written a pattern which matches the text /1/.
Your first conversion is not working because {...} denotes a quantifier, not literal braces, so you need \\\\cite\\{foo\\}. (The four backslashes are because to match a literal \ in a regular expression is \\, and to make that a string literal it is \\\\ — two escaped backslashes.)