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The JGsoft engine, .NET, Java, Perl, PCRE, Python, Ruby and XPath support a variant of the regular expression syntax called free-spacing mode. You can turn on this mode with the (?x) mode modifier, or by turning on the corresponding option in the application or passing it to the regex constructor in your programming language.
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. E.g. 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.
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 ] and \ is the only way to match a space. However. 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.
The XPath flavor does not support comments within the regular expression. The # is always treated as a literal character.
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)
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Page URL: http://www.Regular-Expressions.info/freespacing.html
Page last updated: 21 April 2010
Site last updated: 18 April 2013
Copyright © 2003-2013 Jan Goyvaerts. All rights reserved.
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