I am trying to write a regex that finds the first word in each line that contains the character a. For a string like:

The cat ate the dog
and the mouse

The expression should find cat and
So far, I have:

/\b\w*a\w*\b/g

However this will return every match in each line, not just the first match (cat ate and).
What is the easiest way to only return the first occurrence?

CodePudding user response：

Assuming you are onluy looking for words without numbers and underscores (\w would include those), I'd advise to maybe use:

(?i)^.*?(?<!\S)([b-z]*a[a-z]*)(?!\S)

And use whatever is in the 1st capture group. See an online demo. Or, if supported:

(?i)^.*?\K(?<!\S)[b-z]*a[a-z]*(?!\S)

See an online demo.

Please note that I used lookaround to assert that the word is not inbetween anything other than whitespace characters. You may also use word-boundaries if you please and swap those lookarounds for \b. Also, depending on your application you can probably scratch the inline case-insensitive switch to a 'flag'. For example, if you happen to use JavaScript /^.*?(?<!\S)([b-z]*a[a-z]*)(?!\S)/gmi should probably be your option. See for example:

var myString = "The cat ate the dog\nand the mouse";
var myRegexp = new RegExp("^.*?(?<!\S)([b-z]*a[a-z]*)(?!\S)", "gmi");
m = myRegexp.exec(myString);
while (m != null) {
  console.log(m[1])
  m = myRegexp.exec(myString);
}

CodePudding user response：

If you want to match a word using \w you might also use a negated character class matching any character except a or a newline.

Then match a word that consists of at least an a char with word boundaries \b

^[^a\n\r]*\b([^\Wa]*a\w*)

The pattern matches:

• ^ Start of string
• [^a\n\r]*\b Optionally match any character except a or a newline
• ( Capture group 1
  • [^\Wa]*a\w* Optionally match a word character without a, then match a and optional word characters
• ) Close group 1

Regex demo

Using whitespace boundaries on the left and right:

^[^a\n\r]*(?<!\S)([^\Wa]*a\w*)(?!\S)

Regex demo

CodePudding user response：

The text could be matched with the regular expression

(?=(\b[a-z]*a[a-z]*\b)).*\r?\n

with the multiline and case-indifferent flags set. For each match capture group 1 contains the first word (comprised only of letters) in a line that contains an "a". There are no matches in lines that do not contain an "a".

Demo

The expression can be broken down as follows.

(?=                # begin a positive lookahead
  \b               # match a word boundary
  ([a-z]*a[a-z]*)  # match a word containing an "a" and save to
                   # capture group 1
  \b               # match a word boundary
)
.*\r?\n            # match the remainder of the line including the
                   # line terminator