The modifier that allows using whitespace inside some parts of the pattern to format it for better readability and to allow comments starting with
/(?x)^ # start of string (?=\D*\d) # the string should contain at least 1 digit (?!\d+$) # the string cannot consist of digits only \# # the string starts with a hash symbol [a-zA-Z0-9]+ # the string should have 1 or more alphanumeric symbols $ # end of string /
Example of a string:
#word1here. Note the
# symbol is escaped to denote a literal
# that is part of a pattern.
Unescaped white space in the regular expression pattern is ignored, escape it to make it a part of the pattern.
Usually, the whitespace inside character classes (
[...]) is treated as a literal whitespace, except in Java.
Also, it is worth mentioning that in PCRE, .NET, Python, Ruby Oniguruma, ICU, Boost regex flavors one can use
(?#:...) comments inside the regex pattern.