PHP - How to write a regex to check that a string only contains certain characters?

Question

I have little to no experience in writing regular expressions. How would I go about checking that a string contains only zeros, spaces, hyphens, and colons? Thanks!

riwalk · Accepted Answer · 2012-10-11 19:36:06Z

3

You should get good performance using a simple regex (no forward lookups):

^[0 :-]++$

Breaking it down:

^ recognizes the beginning of the input
[] means that any character within the brackets matches.
+ means that the preceding (the brackets) must match 1 or more times. ++ makes it possesive, improving performance.
$ recognizes the end of the input

edited Oct 11, 2012 at 19:36

answered Oct 11, 2012 at 19:22

riwalk

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6 Comments

riwalk Over a year ago

@AlixAxel, how do you figure? I've never seen a ++ operator before.

Jenny Shoars Over a year ago

Thanks. How does the regex distinguish between the hyphen being used as meaning between (like in "a-z") and being used as meaning itself?

Alix Axel Over a year ago

@JennyShoars: When it's at the start/end of the list - is just another character.

Jenny Shoars Over a year ago

Makes sense, but are there any other special characters that need this special position to be considered just the character that they are? And how do you resolve multiple of these then?

riwalk Over a year ago

@AlixAxel, ah! So ++ behaves exactly as + does in lex/flex. Learn something new every day

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Michael Berkowski · Accepted Answer · 2012-10-11 19:22:21Z

3

/^[0\s:-]+$/

^ = start of string
[0\s:-]+ = one or more zeros, spaces, hyphens, colon. The + means one or more, \s is any whitespace character, which may include line breaks and tabs.
$ = end of string

Since the pattern is anchored between ^ and $, no characters other than those in the [] character class will match.

If instead of any whitespace character, you permit only a literal space, use:

/^[0 :-]+$/

answered Oct 11, 2012 at 19:22

Michael Berkowski

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Comments

Rayshawn · Accepted Answer · 2012-10-11 19:21:10Z

1

You can use a range.

^[0 \-:]{1,}$

answered Oct 11, 2012 at 19:21

Rayshawn

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2 Comments

Alix Axel Over a year ago

{1,} is the same as +. Also, you wouldn't need to escape - if you placed it at the beginning or ending of the list.

Rayshawn Over a year ago

true but you should know what characters you need to escape.

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