## Regexp

Regexp represents a regular expression.

### Parameters

Attribute | Description | Required
--- | --- | ---
pattern | regular expression pattern | Yes
refid | Makes this `regexp` a reference to a `regexp` defined elsewhere. If specified no other attributes or nested elements are allowed. | No

### Examples

    <regexp id="myregexp" pattern="alpha(.+)beta"/>

Defines a regular expression for later use with id myregexp.

    <regexp refid="myregexp"/>

Use the regular expression with id myregexp.

### Choice of regular expression implementation

Apache Ant comes with wrappers for the `java.util.regex` package, jakarta-regexp and jakarta-ORO, see installation dependencies concerning the supporting libraries.

The property `ant.regexp.regexpimpl` governs which regular expression implementation will be chosen. Possible values for this property are:

- `org.apache.tools.ant.util.regexp.Jdk14RegexpRegexp`
- `org.apache.tools.ant.util.regexp.JakartaOroRegexp`
- `org.apache.tools.ant.util.regexp.JakartaRegexpRegexp`

It can also be another implementation of the interface `org.apache.tools.ant.util.regexp.Regexp`. If `ant.regexp.regexpimpl` is not defined, Ant uses Jdk14Regexp as this is always available.

There are cross-platform issues for matches related to line terminator. For example if you use $ to anchor your regular expression on the end of a line the results might be very different depending on both your platform and the regular expression library you use. It is highly recommended that you test your pattern on both Unix and Windows platforms before you rely on it.

- Jakarta Oro defines a line terminator as \n and is consistent with Perl.
- Jakarta RegExp uses a system-dependent line terminator.
- `java.util.regex` uses \n, \r\n, \u0085, \u2028, \u2029 as a default but is configured in the wrapper to use only \n (UNIX_LINE)

We used to recommend that you use Jakarta ORO but since its development has been retired Java's built-in regex package is likely the best choice going forward.

### Usage

The following tasks and types use the Regexp type:

- ReplaceRegExp task
- LineContainsRegexp filter

These string filters also use the mechanism of regexp to choose a regular expression implementation:

- ContainsRegex string filter
- ReplaceRegex string filter
- filename selector
- name resource selector
- containsregexp selector