Replacement Text Reference

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The table below compares the various tokens that the various tools and languages discussed on this website recognize in the replacement text during search-and-replace operations.

The list of replacement text flavors is not the same as the list of regular expression flavors in the regex features comparison. The reason is that the replacements are not made by the regular expression engine, but by the tool or programming library providing the search-and-replace capability. The result is that tools or languages using the same regex engine may behave differently when it comes to making replacements. E.g. The PCRE library does not provide a search-and-replace function. All tools and languages implementing PCRE use their own search-and-replace feature, which may result in differences in the replacement text syntax. So these are listed separately.

To make the table easier to read, I did group tools and languages that use the exact same replacement text syntax. The labels for the replacement text flavors are only relevant in the table below. E.g. the .NET framework does have built-in search-and-replace function in its Regex class, which is used by all tools and languages based on the .NET framework. So these are listed together under ".NET".

Note that the escape rules below only refer to the replacement text syntax. If you type the replacement text in an input box in the application you're using, or if you retrieve the replacement text from user input in the software you're developing, these are the only escape rules that apply. If you pass the replacement text as a literal string in programming language source code, you'll need to apply the language's string escape rules on top of the replacement text escape rules. E.g. for languages that require backslashes in string literals to be escaped, you'll need to use "\\1" instead of "\1" to get the first backreference.

A flavor can have four levels of support (or non-support) for a particular token:

RegexBuddy Get RegexBuddy to easily work with and convert between any of these regular expression flavors.
Syntax Using Backslashes
FeatureJGsoft.NETJavaPerlECMAPythonRubyTclPHP eregPHP pregREAL basicOraclePost
gres		XPathR
\& (whole regex match) YES no no no no no YES no no no no no YES error no
\0 (whole regex match) YES no no no no no YES YES YES YES YES no no error no
\1 through \9 (backreference) YES no no depr. no YES YES YES YES YES YES YES YES error YES
\10 through \99 (backreference) YES no no no no YES no no no YES YES no no error no
\10 through \99 treated as \1 through \9 (and a literal digit) if fewer than 10 groups YES n/a n/a n/a n/a no n/a n/a n/a no no n/a n/a error n/a
\g<group> (named backreference) YES no no no no YES no no no no no no no error no
\` (backtick; subject text to the left of the match) YES no no no no no YES no no no no no no error no
\' (straight quote; subject text to the right of the match) YES no no no no no YES no no no no no no error no
\+ (highest-numbered participating group) YES no no no no no YES no no no no no no error no
Backslash escapes one backslash and/or dollar YES no YES YES no YES YES YES YES YES YES YES YES YES YES
Unescaped backslash as literal text YES YES no YES YES YES YES YES YES YES no YES YES error no
Character Escapes
FeatureJGsoft.NETJavaPerlECMAPythonRubyTclPHP eregPHP pregREAL basicOraclePost
gres		XPathR
\u0000 through \uFFFF (Unicode character) YES no string no string u
string		 no string no no no no no error string
\x{0} through \x{FFFF} (Unicode character) YES no no string no no no no no no no no no error no
\x00 through \xFF (ASCII character) YES no no string string non-r
string		 no string string string YES no no error string
Syntax Using Dollar Signs
FeatureJGsoft.NETJavaPerlECMAPythonRubyTclPHP eregPHP pregREAL basicOraclePost
gres		XPathR
$& (whole regex match) YES YES error YES YES no no no no no YES no no error no
$0 (whole regex match) YES YES YES no no no no no no YES YES no no YES no
$1 through $9 (backreference) YES YES YES YES YES no no no no YES YES no no YES no
$10 through $99 (backreference) YES YES YES YES YES no no no no YES YES no no YES no
$10 through $99 treated as $1 through $9 (and a literal digit) if fewer than 10 groups YES no YES no YES n/a n/a n/a n/a no no n/a n/a YES n/a
${1} through ${99} (backreference) YES YES error YES no no no no no YES no no no error no
${group} (named backreference) YES YES error no no no no no no no no no no error no
$` (backtick; subject text to the left of the match) YES YES error YES YES no no no no no YES no no error no
$' (straight quote; subject text to the right of the match) YES YES error YES YES no no no no no YES no no error no
$_ (entire subject string) YES YES error YES IE only no no no no no no no no error no
$+ (highest-numbered participating group) YES no error YES no no no no no no no no no error no
$+ (highest-numbered group in the regex) no YES error no IE and Firefox no no no no no no no no error no
$$ (escape dollar with another dollar) YES YES error no YES no no no no no no no no error no
$ (unescaped dollar as literal text) YES YES error no YES YES YES YES YES YES buggy YES YES error YES
Tokens Without a Backslash or Dollar
FeatureJGsoft.NETJavaPerlECMAPythonRubyTclPHP eregPHP pregREAL basicOraclePost
gres		XPathR
& (whole regex match) no no no no no no no YES no no no no no no no
General Replacement Text Behavior
FeatureJGsoft.NETJavaPerlECMAPythonRubyTclPHP eregPHP pregREAL basicOraclePost
gres		XPathR
Backreferences to non-existent groups are silently removed YES no error YES no error YES YES no YES YES YES YES YES error

Highest-Numbered Capturing Group

The $+ token is listed twice, because it doesn't have the same meaning in the languages that support it. It was introduced in Perl, where the $+ variable holds the text matched by the highest-numbered capturing group that actually participated in the match. In several languages and libraries that intended to copy this feature, such as .NET and JavaScript, $+ is replaced with the highest-numbered capturing group, whether it participated in the match or not.

E.g. in the regex a(\d)|x(\w) the highest-numbered capturing group is the second one. When this regex matches a4, the first capturing group matches 4, while the second group doesn't participate in the match attempt at all. In Perl, $+ will hold the 4 matched by the first capturing group, which is the highest-numbered group that actually participated in the match. In .NET or JavaScript, $+ will be substituted with nothing, since the highest-numbered group in the regex didn't capture anything. When the same regex matches xy, Perl, .NET and JavaScript will all store y in $+.

Also note that .NET numbers named capturing groups after all non-named groups. This means that in .NET, $+ will always be substituted with the text matched by the last named group in the regex, whether it is followed by non-named groups or not, and whether it actually participated in the match or not.

RegexBuddy Get RegexBuddy to easily work with and convert between any of these regular expression flavors.

| Tutorial | Tools & Languages | Examples | Books & Reference |

| Regular Expr. Cookbook | Teach Yourself Reg. Expr. | Mastering Regular Expr. | Java Regular Expressions | Oracle Regular Expr. | Regular Expr. Pocket Ref. | Regular Expr. Recipes | Regex Recipes for Windows |

| Basic Regex Syntax | Advanced Regex Syntax | Unicode-Specific Syntax | Flavor-Specific Syntax | Flavor Comparison | Replacement Syntax |