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Replacement Text Reference

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:

A "YES" in the table below indicates the token will be substituted.
A "no" indicates the token will remain in the replacement as literal text. Note that languages that use variable interpolation in strings may still replace tokens indicated as unsupported below, if the syntax of the token corresponds with the variable interpolation syntax. E.g. in Perl, $0 is replaced with the name of the script.
The "string" label indicates that the syntax is supported by string literals in the language's source code. For languages like PHP that have interpolated (double quotes) and non-interpolated (single quotes) variants, you'll need to use the interpolated string style. String-level support also means that the character escape won't be interpreted for replacement text typed in by the user or read from a file.
Finally, "error" indicates the token will result in an error condition or exception, preventing any replacements being made at all.

JGsoft: This flavor is used by the JGsoft products, including PowerGREP, EditPad Pro and AceText.
.NET: This flavor is used by programming languages based on the Microsoft .NET framework versions 1.x, 2.0 or 3.0. It is generally also the regex flavor used by applications developed in these programming languages.
Java: The regex flavor of the java.util.regex package, available in the Java 4 (JDK 1.4.x) and later. A few features were added in Java 5 (JDK 1.5.x) and Java 6 (JDK 1.6.x). It is generally also the regex flavor used by applications developed in Java.
Perl: The regex flavor used in the Perl programming language, as of version 5.8.
ECMA (JavaScript): The regular expression syntax defined in the 3rd edition of the ECMA-262 standard, which defines the scripting language commonly known as JavaScript. The VBscript RegExp object, which is also commonly used in VB 6 applications uses the same implementation with the same search-and-replace features. However, VBScript and VB strings don't support \xFF and \uFFFF escapes.
Python: The regex flavor supported by Python's built-in re module.
Ruby: The regex flavor built into the Ruby programming language.
Tcl: The regex flavor used by the regsub command in Tcl 8.2 and 8.4, dubbed Advanced Regular Expressions in the Tcl man pages. wxWidgets uses the same flavor.
PHP ereg: The replacement text syntax used by the ereg_replace and eregi_replace functions in PHP.
PHP preg: The replacement text syntax used by the preg_replace function in PHP.
REALbasic: The replacement text syntax used by the ReplaceText property of the RegEx class in REALbasic.
Oracle: The replacement text syntax used by the REGEXP_REPLACE function in Oracle Database 10g.
Postgres: The replacement text syntax used by the regexp_replace function in PostgreSQL.
XPath: The replacement text syntax used by the fn:replace function in XQuery and XPath.
R: The replacement text syntax used by the sub and gsub functions in the R language. Though R supports three regex flavors, it has only one replacement syntax for all three.

Syntax Using Backslashes
Feature	JGsoft	.NET	Java	Perl	ECMA	Python	Ruby	Tcl	PHP ereg	PHP preg	REAL basic	Oracle	Post gres	XPath	R
`\&` (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
Feature	JGsoft	.NET	Java	Perl	ECMA	Python	Ruby	Tcl	PHP ereg	PHP preg	REAL basic	Oracle	Post gres	XPath	R
`\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
Feature	JGsoft	.NET	Java	Perl	ECMA	Python	Ruby	Tcl	PHP ereg	PHP preg	REAL basic	Oracle	Post gres	XPath	R
`$&` (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
`$_` (entire subject string)	YES	YES	error	YES	YES	no	no	no	no	no	no	no	no	error	no
`$+` (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	YES	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
Feature	JGsoft	.NET	Java	Perl	ECMA	Python	Ruby	Tcl	PHP ereg	PHP preg	REAL basic	Oracle	Post gres	XPath	R
`&` (whole regex match)	no	no	no	no	no	no	no	YES	no	no	no	no	no	no	no
General Replacement Text Behavior
Feature	JGsoft	.NET	Java	Perl	ECMA	Python	Ruby	Tcl	PHP ereg	PHP preg	REAL basic	Oracle	Post gres	XPath	R
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.

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