使用 JavaScript 在 Regex 上排除行首或行尾的匹配项

Question

I'm trying to define a regular expression in JavaScript that matches all ocurrences, excluding the ones on the beginning or on the end of a line.

I can exclude the ones on the beginning but not on the end. For example:

const MULTILINE = `
Lorem ipsum dolor sit amet ANNA
ANNA lorem ipsum dolor sit amet
Lorem ipsum dolor ANNA sit amet
`

MULTILINE.match(/ANNA\w+/gm)
// output: ["ANNA_END", "ANNA_BEGIN", "ANNA_MIDDLE"] ok

MULTILINE.match(/(?!^)ANNA\w+/gm))
// output: ["ANNA_END", "ANNA_MIDDLE"] ok

MULTILINE.match(/ANNA\w+(?!$)/gm))
// output: ["ANNA_EN", "ANNA_BEGIN", "ANNA_MIDDLE"] fail
// expected: ["ANNA_BEGIN", "ANNA_MIDDLE"]

As seen, it correctly identifies my last string, but extracts the last character (as if $ was being replaced by another \\d expression).

I've read lots of documentation an tried several variations such as MULTILINE.match(/ANNA\\w+(?!ANNA\\w+$)/gm)) but without success.

Any help here? :)

Answer 1

The ANN_END returns ANN_EN match because (?!$) lookahead, when failing, makes the engine backtrack, and as the pattern right before (?!$) is \\w+ , a + quantified pattern, the backtracking enables a match to complete before the end of string. See this demo and pay attention at the red arrow that show backtracking at Step 9:

To disallow this partial word matching, you may add a word boundary, \\b , or another lookahead, (?!\\w) .

The complete solution to match ANNA\\w+ not at the start/end of the string will look like

/(?!^)\bANNA\w+\b(?!$)/gm

See the regex demo .

Details

• (?!^) - a negative lookahead that fails the match if the regex index is at the start of the string
• \\b - a word boundary
• ANNA - a substring
• \\w+ - one or more word chars
• \\b - a word boundary
• (?!$) - a negative lookahead that fails the match if the regex index is at the end of the string.

JS demo:

 const MULTILINE = `Lorem ipsum dolor sit amet ANNA_END ANNA_BEGIN lorem ipsum dolor sit amet Lorem ipsum dolor ANNA_MIDDLE sit amet`; console.log(MULTILINE.match(/(?!^)\\bANNA\\w+\\b(?!$)/gm));