|Easily create and understand regular expressions today. |
Compose and analyze regex patterns with RegexBuddy's easy-to-grasp regex blocks and intuitive regex tree, instead of or in combination with the traditional regex syntax. Developed by the author of this website, RegexBuddy makes learning and using regular expressions easier than ever. Get your own copy of RegexBuddy now
In free-spacing mode, whitespace between regular expression tokens is ignored. Whitespace includes spaces, tabs and line breaks. Note that only whitespace between tokens is ignored. a b c is the same as abc in free-spacing mode. But \ d and \d are not the same. The former matches d, while the latter matches a digit. \d is a single regex token composed of a backslash and a "d". Breaking up the token with a space gives you an escaped space (which matches a space), and a literal "d".
Likewise, grouping modifiers cannot be broken up. (?>atomic) is the same as (?> ato mic ) and as ( ?>ato mic). They all match the same atomic group. They're not the same as (? >atomic). In fact, the latter will cause a syntax error. The ?> grouping modifier is a single element in the regex syntax, and must stay together. This is true for all such constructs, including lookaround, named groups, etc.
A character class is also treated as a single token. [abc] is not the same as [ a b c ]. The former matches one of three letters, while the latter matches those three letters or a space. In other words: free-spacing mode has no effect inside character classes. Spaces and line breaks inside character classes will be included in the character class. This means that in free-spacing mode, you can use \ or [ ] to match a single space. Use whichever you find more readable. The hexadecimal escape \x20 also works, of course.
Java, however, does not treat a character class as a single token in free-spacing mode. Java does ignore whitespace and comments inside character classes. So in Java's free-spacing mode, [abc] is identical to [ a b c ]. To add a space to a character class, you'll have to escape it with a backslash. But even in free-spacing mode, the negating caret must appear immediately after the opening bracket. [ ^ a b c ] matches any of the four characters ^, a, b or c just like [abc^] would. With the negating caret in the proper place, [^ a b c ] matches any character that is not a, b or c.
Another feature of free-spacing mode is that the # character starts a comment. The comment runs until the end of the line. Everything from the # until the next line break character is ignored.
Java is the only flavor that treats # as the start of a comment inside character classes in free-spacing mode. The comment runs until the end of the line, so you can use a ] to close a comment. All other flavors treat # as a literal inside character classes.
Putting it all together, I could clarify the regex to match a valid date by writing it across multiple lines as:
# Match a 20th or 21st century date in yyyy-mm-dd format
(19|20)\d\d # year (group 1)
[- /.] # separator
(0[1-9]|1) # month (group 2)
[- /.] # separator
(0[1-9]|[0-9]|3) # day (group 3)
Many flavors also allow you to add comments to your regex without using free-spacing mode. The syntax is (?#comment) where "comment" can be whatever you want, as long as it does not contain a closing parenthesis. The regex engine ignores everything after the (?# until the first closing parenthesis.
Of the flavors discussed in this tutorial, all flavors that support comment in free-spacing mode, except Java and Tcl, also support (?#comment). The flavors that don't support comments in free-spacing mode or don't support free-spacing mode at all also don't support (?#comment).
Did this website just save you a trip to the bookstore? Please make a donation to support this site, and you'll get a lifetime of advertisement-free access to this site! Credit cards, PayPal, and Bitcoin gladly accepted.
Page URL: http://www.regular-expressions.info/freespacing.html
Page last updated: 03 June 2016
Site last updated: 19 August 2016
Copyright © 2003-2016 Jan Goyvaerts. All rights reserved.
|Table of Contents|
|Regex Engine Internals|
|Character Class Subtraction|
|Character Class Intersection|
|Shorthand Character Classes|
|Grouping & Capturing|
|Backreferences, part 2|
|Branch Reset Groups|
|Free-Spacing & Comments|
|Lookahead & Lookbehind|
|Lookaround, part 2|
|Keep Text out of The Match|
|Recursion & Capturing|
|Recursion & Backreferences|
|Recursion & Backtracking|
|POSIX Bracket Expressions|
|Regular Expressions Quick Start|
|Regular Expressions Tutorial|
|Replacement Strings Tutorial|
|Applications and Languages|
|Regular Expressions Examples|
|Regular Expressions Reference|
|Replacement Strings Reference|
|About This Site|
|RSS Feed & Blog|