IT. Expert System.

REGEX

Lookahead & Lookbehind


Lookahead & Lookbehind

Also known as "lookarounds" and "zero-width assertions." They will match characters, but then give up the match and only return the result with or without a match, which is why they are called "assertions". They can be positive or negative.

It is important to note that "lookarounds" do not consume characters in the string, but only assert whether a match is possible or not.

Lookahead

A positive lookahead matches something that is followed by something else. Conversely, a negative lookahead matches something not followed by something else.

These lookahead constructs have the following forms:

(?=regexp)

is the positive lookahead form, and ...

(?!regexp)

is the negative lookahead form.

Value Definition
regexp Specifies any valid regular expression to lookahead for

Lookbehind

Lookbehind differs from the lookahead in that it works backwards by telling the regex engine to move backwards in the string. A lookbehind can be used anywhere in the regex.

These lookbehind constructs have the following forms:

(?<=regexp)

is the positive lookahead form, and ...

(?<!regexp)

is the negative lookahead form.

Value Definition
regexp Specifies any valid regular expression to lookbehind for

Support Note:
Only .NET supports a full regex inside the lookbehind. Perl and Python only allow fixed-length strings. JavaScript and Ruby do not support lookbehind.

For example, if you want to find a word not ending with an "s", you could use: \b\w+(?<!s)\b



Content

Android Reference

Java basics

Java Enterprise Edition (EE)

Java Standard Edition (SE)

SQL

HTML

PHP

CSS

Java Script

MYSQL

JQUERY

VBS

REGEX

C

C++

C#

Design patterns

RFC (standard status)

RFC (proposed standard status)

RFC (draft standard status)

RFC (informational status)

RFC (experimental status)

RFC (best current practice status)

RFC (historic status)

RFC (unknown status)

IT dictionary

License.
All information of this service is derived from the free sources and is provided solely in the form of quotations. This service provides information and interfaces solely for the familiarization (not ownership) and under the "as is" condition.
Copyright 2016 © ELTASK.COM. All rights reserved.
Site is optimized for mobile devices.
Downloads: 196 / 158797884. Delta: 0.00297 с