Most modern regex flavors support a variant of the regular expression syntax called free-spacing mode. This mode allows for regular expressions that are much easier for people to read. Of the flavors discussed in this tutorial, only std::regex, XML Schema and the POSIX and GNU flavors don’t support it. Plain JavaScript doesn’t either, but XRegExp does. The mode is usually enabled by setting an option or flag outside the regex. With flavors that support mode modifiers, you can put (?x) the very start of the regex to make the remainder of the regex free-spacing.

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. abc 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".

Flavors differ in how they handle groups that are opened with multiple characters. (?:group) is always the same as (?:group) in free-spacing mode. But you should not put spaces in the middle of the (?: that opens the group. Only a few flavors like Java and ICU recognize ( ?:group) and (? :group) as a non-capturing group. All other flavors treat the latter two regexes as errors. But even with Java and ICU it is best not to have those spaces as they may confuse the next developer who has to work on your code. This is true for all such constructs, including atomic groups, named groups, lookaround, etc.

Exactly which spaces and line breaks are ignored depends on the regex flavor. Oracle only ignores the ASCII space and line feed. All other flavors discussed in this tutorial (besides XPath—see below) ignore the ASCII space, tab, line feed, carriage return, and form feed characters. JGsoft V2, ICU, Boost, and Tcl are the only flavors that ignore all Unicode spaces and line breaks. JGsoft V1 almost does but misses the next line control character (U+0085). Perl always treats non-ASCII spaces as literals. Perl 5.22 and later ignore non-ASCII line breaks. Perl 5.16 and prior treat them as literals. Perl 5.18 and 5.20 treated unescaped non-ASCII line breaks as errors in free-spacing mode to give developers a transition period.

Free-Spacing in Character Classes

A character class is generally 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 and ICU, however, do not treat a character class as a single token in free-spacing mode. They do ignore spaces, line breaks, and comments inside character classes. So in Java’s and ICU’s free-spacing mode, [abc] is identical to [abc]. 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. [^abc] matches any of the four characters ^, a, b or c just like [abc^] would. With the negating caret in the proper place, [^abc] matches any character that is not a, b or c.

Perl 5.26 offers limited free-spacing within character classes as an option. The /x flag enables free-spacing outside character classes only, as in previous versions of Perl. The double /xx flag additionally makes Perl 5.26 treat unescaped spaces and tabs inside character classes as free whitespace. Line breaks are still literals inside character classes. PCRE2 10.30 supports the same /xx mode as Perl 5.26 if you pass the flag PCRE2_EXTENDED_MORE to pcre2_compile().

Perl 5.26 and PCRE 10.30 also add a new mode modifier (?xx) which enables free-spacing both inside and outside character classes. (?x) turns on free-spacing outside character classes like before, but also turns off free-spacing inside character classes. (?-x) and (?-xx) both completely turn off free-spacing. PHP 7.3.0 and R 4.0.0 also support these mode modifiers.

Java treats the ^ in [^a] as a literal. Even when spaces are ignored they still break the special meaning of the caret in Java. ICU treats ^ in [^a] as a negation caret. Perl 5.26, and PCRE2 10.30 do the same in (?xx) mode. ICU, Perl, and PCRE2 still consider the caret to be at the start of the character class even if there is free whitespace before it.

XPath Strips Whitespace First

So far this topic hasn’t mentioned XPath. XPath functions that use a regular expression also accept an optional $flags parameter. If this parameter includes "x" then XPath strips all ASCII spaces, tabs, carriage returns, and line feeds from the string that you pass as your regex before actually compiling your regex. This is different from the other flavors which ignore certain whitespace while compiling your regex, which is why they still allow whitespace to be escaped with a backslash, for example.

But, with XPath, \ d is the same as \d in free-spacing mode. XPath does not, however, remove whitespace between a pair of square brackets. So whitespace inside character classes is not affected. XPath does not support comments inside regular expressions.

Comments in Free-Spacing Mode

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 feed character is ignored. Most flavors do not recognize any other line break characters as the end of a comment, even if they recognize other line breaks as free whitespace or allow anchors to match at other line breaks. Since a line feed terminates a comment, a CRLF line break does too. Java and ICU also allow a lone CR to end a comment. JGsoft V2 is the only flavor that recognizes all Unicode line breaks. Boost misses the vertical tab.

XPath and Oracle do not support comments within the regular expression, even though they have a free-spacing mode. They always treat # as a literal character.

Java and ICU are 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. That includes Perl 5.26 and PCRE2 10.30 in (?xx) mode.

Putting it all together, the regex to match a valid date can be clarified by writing it across multiple lines:

# Match a 20th or 21st century date in yyyy-mm-dd format((?:19|20)\d\d)# year (group 1)[- /.]# separator(0[1-9]|1[012])# month (group 2)[- /.]# separator(0[1-9]|[12][0-9]|3[01])# day (group 3)

RegexBuddy makes regular expressions more readable with syntax coloringRegexBuddy makes regular expressions more readable with syntax coloring

Comments Without Free-Spacing

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. A line break does not end such a comment.

Of the flavors discussed in this tutorial, all flavors that support comments 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 don’t support (?#comment) either.