Regular expression which matches a pattern or is an empty string

Question

I have the following Regular Expression which matches an email address format      w   -      w -       a-zA-Z      This is used for validation with a form using JavaScript  However  this is an optional field  Therefore how can I change this regex to match an email address format  or an empty string   From my limited regex knowledge  I think  b matches an empty string  and   means  Or   so I tried to do the following  but it didn t work      w   -      w -       a-zA-Z     b

User · Answer

b matches a word boundary  I think you can use    for empty string

User · Answer

I m not sure why you d want to validate an optional email address  but I d suggest you use          s       s      meaning            empty string           or           beginning of string     s     any character but   or whitespace                s             end of string   You won t stop fake emails anyway  and this way you won t stop valid addresses

User · Answer

An alternative would be to place your regexp in non-capturing parentheses  Then make that expression optional using the   qualifier  which will look for 0  i e  empty string  or 1 instances of the non-captured group   For example        some regexp       In your case the regular expression would look something like this          w   -      w -       a-zA-Z         No    or  operator necessary   Here is the Mozilla documentation for JavaScript Regular Expression syntax

User · Answer

To match pattern or an empty string  use     pattern   Explanation     and   are the beginning and end of the string anchors respectively    is used to denote alternates  e g  this that    References   regular-expressions info Anchors and Alternation     On  b   b in most flavor is a  word boundary  anchor  It is a zero-width match  i e  an empty string  but it only matches those strings at very specific places  namely at the boundaries of a word   That is   b is located    Between consecutive  w and  W  either order     i e  between a word character and a non-word character  Between   and  w   i e  at the beginning of the string if it starts with  w  Between  w and     i e  at the end of the string if it ends with  w    References   regular-expressions info Word Boundaries     On using regex to match e-mail addresses  This is not trivial depending on specification   Related questions    What is the best regular expression for validating email addresses    Regexp recognition of email address hard    How far should one take e-mail address validation

[javascript] Regular expression which matches a pattern, or is an empty string

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