Strip words beginning with a specific letter from a sentence using regex

I'm not sure how to use regular expressions in a function so that I could grab all the words in a sentence starting with a particular 开发者_开发技巧letter. I know that I can do:

word =~ /^#{letter}/ 

to check if the word starts with the letter, but how do I go from word to word. Do I need to convert the string to an array and then iterate through each word or is there a faster way using regex? I'm using ruby so that would look like:

matching_words = Array.new
sentance.split(" ").each do |word|
  matching_words.push(word) if word =~ /^#{letter}/ 
end

Scan may be a good tool for this:

#!/usr/bin/ruby1.8

s = "I think Paris in the spring is a beautiful place"
p s.scan(/\b[it][[:alpha:]]*/i)
# => ["I", "think", "in", "the", "is"]

• \b means 'word boundary."
• [:alpha:] means upper or lowercase alpha (a-z).

You can use \b. It matches word boundaries--the invisible spot just before and after a word. (You can't see them, but oh they're there!) Here's the regex:

/\b(a\w*)\b/

The \w matches a word character, like letters and digits and stuff like that.

You can see me testing it here: http://rubular.com/regexes/13347

Similar to Anon.'s answer:

/\b(a\w*)/g

and then see all the results with (usually) $n, where n is the n-th hit. Many libraries will return /g results as arrays on the $n-th set of parenthesis, so in this case $1 would return an array of all the matching words. You'll want to double-check with whatever library you're using to figure out how it returns matches like this, there's a lot of variation on global search returns, sadly.

As to the \w vs [a-zA-Z], you can sometimes get faster execution by using the built-in definitions of things like that, as it can easily have an optimized path for the preset character classes.

The /g at the end makes it a "global" search, so it'll find more than one. It's still restricted by line in some languages / libraries, though, so if you wish to check an entire file you'll sometimes need /gm, to make it multi-line

If you want to remove results, like your title (but not question) suggests, try:

    /\ba\w*//g

which does a search-and-replace in most languages (/<search>/<replacement>/). Sometimes you need a "s" at the front. Depends on the language / library. In Ruby's case, use:

string.gsub(/(\b)a\w*(\b)/, "\\1\\2")

to retain the non-word characters, and optionally put any replacement text between \1 and \2. gsub for global, sub for the first result.

/\ba[a-z]*\b/i

will match any word starting with 'a'.

The \b indicates a word boundary - we want to only match starting from the beginning of a word, after all.

Then there's the character we want our word to start with.

Then we have as many as possible letter characters, followed by another word boundary.

To match all words starting with t, use:

\bt\w+

That will match test but not footest; \b means "word boundary".

Personally i think that regex is overkill for this application, simply running a select is more than capable of solving this particular problem.

"this is a test".split(' ').select{ |word| word[0,1] == 't' } 

result => ["this", "test"]

or if you are determined to use regex then go with grep

"this is a test".split(' ').grep(/^t/)

result => ["this", "test"]

Hope this helps.