How can we use regular expressions to identify strings that do not begin with a letter or number?

Imagine having the string below:

Lorem #test ^%#test ipsum#test dolor #test sit #testamet.
      ^^^^^                          ^^^^^

What regular expression should be used to specifically select the highlighted #test instances? It is important to only choose instances that are not part of another word or contain additional characters.

If searching for just test, you could use /\btest\b/g. However, it appears that \b only works when the text begins with a word character. Any ideas on how to approach this?

javascriptregex

Answer №1

In JavaScript, the lack of lookbehind assertions means there is no exact substitute for \b.

However, if you're okay with allowing a preceding space character to be part of the match, you can use:

/(?:^|\s)#test\b/m

This pattern will match "#test" at the beginning of a line or " #test" otherwise.

If you prefer different characters besides spaces to define your "word boundary", you can specify them. For instance, using (?:^|\W)#test\b/m will match ",#test" in "foo,#test,foo". Unfortunately, you cannot exclude the comma from the match. However, by utilizing:

var match = /(?:^|\W)(#test)\b/m.exec(subject);
if (match != null) {
    result = match[1];
}

You can get closer to achieving your desired outcome.

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