Regex for Input Validation in PHP

Question

I need a Regex to accept the input only following possible cases like

1.It start with characters only [a-zA-Z]

2.It may contains numbers but it may not repeated 3 or moretimes.

Example:

akjsjdfljsfjl133113 Valid

123123sfsf not valid

asfsdf1111asdf notvalid adf111 not valid

I have tried this code

$input_line="sfjs123232";
preg_match("/^[a-zA-Z](\d)\1{2,}/", $input_line, $output_array);

FYI, they may be gently pulling your leg: strictly speaking, repeated three times means that it is present four or more times (the first instance, then the three repeats.) :) — zx81
– zx81, Commented May 13, 2014 at 4:43
Is a121212121212 acceptable? The digits 1 and 2 are repeated many times in the string. If it is acceptable and you only meant not repeated consecutively, then I misunderstood the question. — zx81
– zx81, Commented May 13, 2014 at 5:00

hwnd · Accepted Answer · 2014-05-13 05:13:28Z

4

You can use a Negative Lookahead here.

^[a-zA-Z]+(?:(?!.*(\d)\1{2,}).)*$

See Live demo

Regular expression

^                the beginning of the string
[a-zA-Z]+        any character of: 'a' to 'z', 'A' to 'Z' (1 or more times)
(?:              group, but do not capture (0 or more times)
 (?!             look ahead to see if there is not:
  .*             any character except \n (0 or more times)
  (              group and capture to \1:
   \d            digits (0-9)
  )              end of \1
   \1{2,}        what was matched by capture \1 (at least 2 times)
  )              end of look-ahead
  .              any character except \n
 )*              end of grouping
$                before an optional \n, and the end of the string

edited May 13, 2014 at 5:13

answered May 13, 2014 at 4:49

hwnd

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3 Comments

zx81 Over a year ago

What about a121212121212? Your regex matches that. But I probably misunderstood the problem: I thought it was about digits not appearing 3 times anywhere in the string. This is the problem that I don't know how to solve without using infinite lookbehinds (not available in PHP).

zx81 Over a year ago

I see that your interpretation was right: he said that akjsjdfljsfjl133113 is a valid string, even though it has 3 1s. +1 :)

hwnd Over a year ago

@zx81 He meant the same digit repeated 3 times as in abc111def

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