Regular Expression to match a word

Question

How do I create a regular expression to match a word at the beginning of a string. We are looking to match stop at the beginning of a string and anything can follow it.

For example the expression should match:

stop
stop random
stopping

Thanks.

Answer 1

If you wish to match only lines beginning with stop use

^stop

If you wish to match lines beginning with the word stop, this is, followed by a space

^stop\s

Or, if you wish to match lines beginning with the word stop but followed by either a space or any other non word character you can use (your regex flavor permitting)

^stop\W

On the other hand, what follows matches a word at the beginning of a string on most regex flavors (in these flavors \w matches the opposite of \W)

^\w

If your flavor does not have the \w shortcut, you can use

^[a-zA-Z0-9]+

Be wary that this second idiom will only match letters and numbers, no symbol whatsoever.

Check your regex flavor manual to know what shortcuts are allowed and what exactly do they match (and how do they deal with Unicode.)

Answer 2

/stop([a-zA-Z])+/

Will match any stop word (stop, stopped, stopping, etc)

However, if you just want to match "stop" at the start of a string

/^stop/

will do :D

Answer 3

Try this:

/^stop.*$/

Explanation:

• / charachters delimit the regular expression (i.e. they are not part of the Regex per se)
• ^ means match at the beginning of the line
• . followed by * means match any character (.), any number of times (*)
• $ means to the end of the line

If you would like to enforce that stop be followed by a whitespace, you could modify the RegEx like so:

/^stop\s+.*$/

• \s means any whitespace character
• + following the \s means there has to be at least one whitespace character following after the stop word

Note: Also keep in mind that the RegEx above requires that the stop word be followed by a space! So it wouldn't match a line that only contains: stop

Answer 4

If you want to match anything that starts with "stop" including "stop going", "stop" and "stopping" use:

^stop

If you want to match the word stop followed by anything as in "stop going", "stop this", but not "stopped" and not "stopping" use:

^stop\W

Answer 5

I'd advise against a simple regular expression approach to this problem. There are too many words that are substrings of other unrelated words, and you'll probably drive yourself crazy trying to overadapt the simpler solutions already provided.

You'll want at least a naive stemming algorithm (try the Porter stemmer; there's available, free code in most languages) to process text first. Keep this processed text and the preprocessed text in two separate space-split arrays. Make sure each non-alphabetical character also gets its own index in this array. Whatever list of words you're filtering, stem them also.

The next step would be to find the array indices which match to your list of stemmed 'stop' words. Remove those from the unprocessed array, and then rejoin on spaces.

This is only slightly more complicated, but will be much more reliable an approach. If you've got any doubts on the value of a more NLP-oriented approach, you might want to do some research into clbuttic mistakes.